Was driving along quite happily when the car cut out at a set of lights and wouldn't start again. It's turning over but won't fire up.

Got the AA out, fuel going through and it's getting a spark but won't start. He checked the coil and said no power getting through. Turned it over a few times and it started.....then cut out again when i revved it.

Got it towed to the garage but they can't find the problem as it has now decided to start every time. Went to pick it up and now it won't start

Garage say it can't be the coil as it either works or it doesn't. They think it's the immobiliser but surely it wouldn't turn over and pump fuel through if it was?

Any ideas??

Ta