Just done a run up the motorway and noticed that at about 4k revs (in 5th (about 100mph) it is missfiring quite bad, if you back off the throttle it stops missfiring and carries on pulling.
Ive only just picked it up from Vauxhall after a main service, so the plugs are new and the throttle body had been cleaned with carb cleaner. It even did this before the service but i thought it might have cured it by changing the plugs.
Ive found a few other similar threads on this relating to the coil pack, does it sound like this could be the problem? or maybe the plug gap?
Obviously its not often i do 100mph but i want the car to work perfect!