My 2003 Ford Ranger 3.0L V6 79,000 miles has been acting up lately. I took it to a mechanic who could not figure out the problem. It does not like to accelerate over 3000-3500 RPMs. Sometimes it acts as if nothing is wrong. Other times it bucks and kicks, just doesnt like hard acceleration, such as going up hills. Recently replaced the fuel filter and ran an injector cleaner throught the engine. Professional Injection cleaner 2 years ago. I was thining it could be the fuel pump, but its not constant. Any ideas?