So yesterday i came out of my drive in my S go down to 300 yards down the road and the car was reving itself to 2-3 revs then dropping back to idle my foot was off the accelerator pedal and it kept doing that i had to turn off the key.

I then went under the hood and could not see anything wrong so i just thought there could be wrong with the throttle cable which links to the butterfly so i gave the butterfly a few open and closes started the car seemed fine.

only thing i can narrow the problem down to was:

crap fuel from tesco

or.....

someone at the herts meet reving my engine using the butterfly....

 

The valve sticking on the cam cover can cause a simliar problem, give it a clean with WD40.