Hi I have a X18XE1 engine Vectra B. The problem I have is from pulling away the acceleration is sluggish and jerky then all of a sudden it grabs and revs and takes off and I have to put the clutch in and let of the gas to control it. This occurs up to 15 mph after that its all fine. I've not got any engine management lights on and I've taken off the air intake and can see that the butterfly valve is not sticking. Any ideas? TIA