My Vauxhaul Corsa Breeze keeps stuttering. Any idea what it could be?

I've got a 1 litre, T -reg corsa breeze which doesn't cut out but starts to loose power out of the blue when I'm driving. If I keep my foot on the accelerator it still slows down then suddenly picks up, slows down, picks up and so on. So it does some pretty strong stuttering. My ususal mechanic said that he can't find out what it is unless it does it there and then when he test drives it. But as soon as you turn it off, wait for 30 seconds and start it up again, it's fine. So not much chance that it'll do it when he's sitting in it. Any ideas then what he could check for me?