I have a 1999 legacy 2.2 that is sluggish, lacks power and sputters until it reaches about 3300 RPM's, then it's like something kicks in and the cars runs great. If the car is allowed to cool off and you reset the ECU (Unplug the ECU or battery) the issue temperately goes away, but after running for approximately 20 - 30 minutes, the issue slowly returns. Any idea's?

Edited December 27, 201510 yr by gardorr

Any codes? When was the tune up, fuel treatment? And only when cold 3300 is fine? Sounds like a bad head gasket and coolant pressure is making its way into the head.

No codes and it has a fresh rebuild on it. Everything been replaced including the head gaskets.

Once it starts acting up it doesn't stop until you reset the ECU. So even if I let it cool off and then started it back up the issue persists. 

Knock sensor.

I already tried replacing the knock sensor, but it didn't change anything.