Just bought a P reg Escort 1.6 zetec, when i start it it revs up to around 2000-2500, then drops to around 500. It shudders but doesn't cut out and then goes way back up to around 2000 again and the process keeps going. after a bit of driving it seems to happen less frequently, but sometimes happens at traffic lights etc.

Anyone have any ideas or tips?

Cheers.