Whilst driving at a constant speed my Rev's suddenly go through the roof and the car sparts to slow down as if taken out of gear.This only appears to happen when the weather is very cold. I change to a different gear and bring the clutch up but there doesn't appear to be anything there to move the car. If you touch the accelerator the rev's go through the roof again. Only way to re-drive the car is to switch off and re-start the engine again. The car then drives ok. Vauxhall have checked for faults but none listed on log and they cannot get the car to do the same problem.