Episode one is the set-up, and in times gone by it was the episode audiences forgave. Cold starts are tricky, and once upon a time we allowed makers their fumbling beginnings. These days...not so much. Get your bile posted or printed within minutes and ignore episodes two through six. Why the hell not? What great show ever got better after its pilot?

Bile about bile aside, Lab Rats' second episode isn't quite doing the job for either side. It's not so spectacular as to be a vindication for those of us who were pretty taken with episode one, nor is it so poor that it justifies the swift rejection slip many handed out last week.

It is, to be certain, a great relief to have a silly, gaggy sitcom on TV while Graham Linehan beavers away on series three of The IT Crowd - though it has become tediously inevitable to see makers and critics talk about Lab Rats in terms of being 'old fashioned'. Sure, it has a studio audience, but... is that really all it takes? When a format survives from the 1940s to the present day, it's not just 'old' is it? And if it is, was it not already old thirty years ago? I mean... is the internal combustion engine just 'old'?

A commitment to punchlines doesn't make you old fashioned, and going for a silly tone certainly doesn't. Father Ted happened in the mid-90s, not the Dark Ages. When did we decide to punish comedy for trying to be directly, simply funny? (Round about the same time people started using the term 'canned laughter' with reckless abandon and insisting they 'don't need to be told when to laugh', presumably.)

Still that's an outside issue - launching a show on the defensive is as much a product of the market as the makers.

A Donor has some good stuff going for it - Cryogenic freezing is ideal subject matter for a show that last week focussed on the difficulties of human shrinkage during cloning, and the utilising of the cryo-freezer to store food is only good comedy sense - but, as a second episode, it doesn't really capitalise on the best the show has to offer. It doesn't prove itself.

For a start, Lab Rats has two of the best female character/performance combinations I've seen in a comedy for a long while. Selina Cadell (as Dean) and Jo Enright (playing Cara) completely nail the tone of the show, playing their characters as if they were leads in their own, imaginary shows. But this episode makes far too little use of them.

Everyone else, it has to be said, is finding the tone harder to nail. This is especially odd when the lead - Chris Addison - is also the co-writer. He struggles to know whether to play 'natural' or 'big', and kinda misses both (not that either is correct). The often-brilliant Geoff McGivern - slimmed down to the point of being almost unrecognisable - drives at his lines with a JCB, knocking them 'over' rather than 'out of the park', belabouring when he should be breezing through. And Helen Moon's Minty does little with even less.

Thank heavens, then, for guest star Kim Wall as an officious, unpredictable inspector from the Surprise Surprise Department. For two weeks running now Lab Rats has hit gold with its main guests. (I'm ignoring the mostly-too-annoying-to-be-too-funny turn by additional material writer Robin Ince this week.) Last week's Sevan Stephan was guest-priest-in-Father-Ted great, and Wall - again playing as if he's his own main character - finds just the right beat to make the lines sing. ("Three: racist." Well, you really had to be there.)

One of the reasons Lab Rats is easy to criticise is its insistence on opening on a series of bang-bang-bang jokes, puns, musings and insults. An almost Airplane! go-for-the-lot approach that gets risky with an audience in the mix. If the jokes aren't nailed, the (lack of) reaction draws attention to the fact, and the show starts to struggle right out of the gate. The rhythm is just too unforgiving.

It does seem like the makers know this, because the laugh track on the opening scenes have been awfully erratic on both episodes so far. Blatantly the result of retakes and edits, the laughs are there, but weirdly they're not quite in the right place. Which is almost worse than playing to silence.

In fact the technical side of the show has been very iffy. Either the microphones aren't finding the actors fast enough, or the actors keep moving in unrehearsed ways, but either way there's a noticeable rise-and-fall in the sound quality in the middle of busy scenes. Lines too often begin or end clipped, or echo in ways they shouldn't, which is a bit of a killer when the lines are where your laughs are. (Worth noting, too, that A Donor included one poorly-improvised 'tag' at the end of a scene, one crash zoom from a cameraman who clearly didn't quite get to the shot in time, and a clumsy mispronunciation of 'Hedda Gabler' - all of which should have been cut/reshot.)

Still, the pressure to start big thankfully eases off as the episode gets going, and eventually - as with last week - we settle into the stuff that really works, the character quirks and unexpected bizarreness. Alex's interview scene with the inspector - where Addison actually does get to grips with the tone thanks to a back-and-forth rhythm with Wall - was a splendid tableaux of mounting nonsense, and there is something to be said for two characters coincidentally wearing the same outfit... if you time it right. (They did. Or, at least, they got close enough.)

If there's one piece of advice to give to a show that's already been filmed and edited - and there isn't - it's this: calm down. The thing that's annoying people, that's bringing all these 'old fashioned' crits out of the woodwork, is how hard everyone's trying. It's the curse of comedy that everyone wants your very best effort, but they want the whole thing to feel effortless.

The scene transitions are a case in point. Little animated moments intended to keep things bouncy as we ping from one scene to another. Thing is, we rarely ping. You'll only get two or three transitions per episode - which means when they show up, they're a little startling. You have to consciously think - "Oh yes, they do that funny animation stuff."

Stop trying! It worked fine in Third Rock From The Sun because they were doing it eight times in 22 minutes. Twice in 28 minutes isn't part of the grammar - it's a mispronunciation. You don't need to fight so hard to convince us of the tone you're going for.

The best scenes in Lab Rats so far (the light bulb scene, the interrogation, "This chair is an arsehole", "and that's why disabled toilets are unisex") have been the at-ease moments. They just seem to glide past. When everybody seems to be trying awfully hard, somehow we're placed at a distance.

This week's ending is a good example. When the plot was forced against its natural flow to 'do an ending', we got a flat 'overloaded the electrics' moment. Fail. Trying to hard.

But then - ah-ha - two bits of natural-feeling, underplayed business lift the whole thing. We hear the cries of Ince's irritating cryogenic git, once more defrosted, coming from off-screen. Which worked. Then, as is clearly going to be come a bit of a runner, the credits arrive just in time to cut off a character's exclamation of "Fu-" - which worked even better.

Even though, okay, it was a bit old fashioned.