it might be a misfire that only makes itself noticable at higher rpms. my dads galant used to do it from 2.5 to 3. 2 of the injectors were sticking but i just duped a **** ton of injector cleaner into the tank and by the end of the tank it ran fine. on another car i was selling i sprayed the engine down with water to clean it and after that it ran like ****. at lower rpms it ran fine but if you throttled it from about 3 and up it started runnin like **** reason why it had water inside the spark plug well and im sure the distributor cap as well. i never fixed that problem i just got rid of the car but im sure all i had to do was let it evaporate or sum.