Originally Posted by mysterio
.... But there is still remains a hesitation when i am accelerating. Around 2500-3000rpm. ..... There seems to be a engine knock at high RPMs.
This might be what is happening: If your car is working as it should, when it starts to knock, the knock sensor will report that information to the PCM, which will retard the spark advance of the engine drastically - depending on the frequency and amplitude of the sound - up to nearly 20 degrees of spark advance. This is called (I think) knock retard, or KR. Some say that the computer is "pulling timing." When this happens, the car will suddenly feel very sluggish and slow. This feature is meant to adjust for load and fuel octane to eliminate preignition or "spark knock." But if you have a mechanical problem like a loose connecting rod or balance shaft or even valve train noise, the knock sensor doesn't know the difference and retards the spark.
Like Ben said, the TPS throttle position sensor sits on the end of the throttle shaft and senses how far open the throttle is. The one on top is the mass air flow (MAF) meter, and the other one is the idle air control (IAC). If you have a nice idle now, the IAC is probably not the problem. But either of the other two can cause serious driveability issues if they are bad. Consider buying or checking out from your local library, a Chilton'* 28200 car repair guide. It will show you all this stuff and tell you how to test each one to see if it is working right, to improve your chances of replacing the bad part on the first try.