how can i fix my 2002 vauxhall astra sxi from misfireing?

had it put on dinostics and was told it was the coil pack and plugs that need changing si i have had them changed by myself but the problem is still there, when i first start the car its fine runs sweet but when i drive for about 2 mins it starts to misfire, when i have the light put out it seems to be fine for a while and power seems to be there but when im on the motorway and put the throttle down the engine management light flashes and when i ease off it goes out can anyone help me please i have also tryed cataclean to see if that helped but hasnt?????????????

matt
December 2009