check the wiring loom that feeds the coil packs where it runs in the plastic cover over the engine. i had something similar-random misfire and it turned out to be some melted earth wiring(i think,cant quite remember).replaced the wiring and it was ok,in fact thinking about it the earth wire that goes on the last coil pack(no.6) wasnt connected and i presumed that had caused it.
worth a look.....

another problem i had turned out to be a completely blocked cone air filter,car would run but was flat as fuck-changed all sorts including air flow meter,inlet gaskets,injectors cleaned.......it was only when i was putting the filter back on i noticed how blocked it was!!! but it was my track car and i ran it without headlights in.