mg zr think its a misfire?

my mg zr started to misfire so i replaced the plugs to no avail then the 2 coil packs which worked for a while but then my exhaust split which brought the misfire back so a full stainless steel was put on but after a few days i was driving along and the car started to what i think was a very short misfire then the engine management light flashed on then went out, the car is now "misfiring" when revved and driven above 4000 rpm I need help plzzzzzzzzzzz

AT WITS END
July 2008