I think your trottle cable is getting stuck, i may be wrong but im quite sure its that, best way to check is rev the car from the bay and then pull the throttle cable right back to where its supposed to be, if that doesnt fix things then replace the throttle position sensor (£10ish) and see if that fixes things.

Mine sits just under 1000rpm and sometimes revs up a tad but not much and instantly goes back down, the sensor is what i think is up with mine, that and the lambda sensor, im checking both anyway.

http://en.wikipedia.org/wiki/Throttle_position_sensor