I have a problem where the revs on my Corsa increase from 2000rpm to 3000rpm excessively quick and then drop and then eventually settles out. The rev counter needle sort of moves up and down for a few seconds.

Around town the vehicle is ok, it is just when the car is being used on National Speed Limits and on the approach to motorways and acceleration is harder than usual. It is as though the turbo is kicking in and is running faster than the engine.

Car was recently serviced and had a clean bill of health.

I wondered if it was a sticking clutch but have ruled that one out. Checked the oil levels this is ok, as I have heard that over filling can cause the problem with an over revving engine.

Problem occurs when air con is both on and off.

Could it be a sticking throttle response or something else, there are no ECU lights so would assume that it is not electrical. It is also smoking a little more than usual. I have given it a blast down the motorway to see if the problem would clear.

Any ideas?