No Acceleration?

My Vauxhall vectra has no accelertion!!
Been a bit weird for a couple of weeks, go to pull away and it creeps along (not good when your;e trying to whizz across a roundbout) revs on 2000 and nothing when you put your foot down i found if I turned the engine off and started it up again it was fine. However yesterday started it up was a bit juddery revs went to 2000 no acceleration?? tried the switch it off and switch it back on again trick but to no avail
Help, damsel in distress!