1998 Vauxhall Astra slight misfire

hi when i start my car up there is a slight misfire and my rev counter goes up and down as the car gets hotter the misfire gets worse no problems when the car is going been to the vauxhall dealers they put it on the machine but the ecu wouldnt let the computer in an older machine found a fault it was the timing but i thought you couldnt set the timing on these please help as ive spent a fortune on the car