What did you have checked out, and how did you check it?
Is the engine management light on, is the bulb working?
Did this problem start suddenly or get worse over time, was any work done on the car prior to this happening?
From the symptoms I'd guess crankshaft position sensor but without more info it is just a guess.
If fitting a new CKP sensor use a proper Vauxhall part.
vxl
August 2009