Hey I have a 99 Impreza OBS 2.2 auto with 168,000 miles and I am having a really hard time trying to wrap my head around this issue! It has bin getting worse for the past few months and I have no idea what to do. I am getting a really bad dead spot in my throttle mainly when under load i.e accelerating  threw a corner or going up hill. the throttle response is fine for about the first 1/4 of the peddle and then flatt with super slow acceleration until it hits 3200RPM and then it throws you back in your seat. I have replaced plugs, wires,fuel filter, front AF sensor, cleaned the MAF sensor, tranny flush and filter, air filter, and knock sensor. I have no CEL's on or stored and the only one I ever got was for the knock sensor and the car had the problem before that. If I disconnect the battery and wait 15-20 minutes then hook it back up the problem goes away and the car drives fine under any condition then it returns after about 80 miles. Any help will be greatly appreciated!!!!!!!!!!!!! Thanks 

The throttle position sensor won't throw any codes until it is completey shot.

 

It tests easily with an ohmmeter, or it might be cheap enough to just try it on a lark.

did that and it test out at .5 volts

You need to test at all points in the throttle range and make sure the voltage (or ohms if checking ohms with connector disconnected) sweeps with no breaks or sudden jumps.

 

You need to check the center 'wiper' pin to one or both end pins.

Add a can of Techron at 1/8 tank, change your plug wires. Plugs too but don't if you don't know aluminum thread torque behavior.