It Need help on a 4age... Options

[biglipzit]

Ok guys it isnt a 6th gen but a toyota levin with a 4age engine that i am working on... It has some serious problems which I am baffled by. Sometimes it works and sometimes it doesnt... What I mean is sometimes it runs perfect and other times it picks up slowly and has almost no power. The vvti kicks in at 4000rpm like normal all the way up to the 8300rpm rev limiter but then engine light comes on after a while and it wont deliver as much power or rev as fast as it use to. I changed injectors and fuel pump thinking it might be a fuel pressure problem and still nothing. Change fuel pressure regulator and it isnt that. Tried a different ecu and it isnt that either. It wouldnt start the other day after a period of running with the engine light on and i had to disconnect the battery and reset the ecu before it would spark again and it started again and ran perfectly then after a stop to the gas station the engine light was on again and it wouldnt deliver... I thought it could be compression rings and valve seals going but then that wouldnt explain why it only happens sometimes. If it was compression then it wouldnt work sometimes and not work others. I am thinking that maybe the O2 sensor is bad and messing up the ecu and causing the loss of power and revs... Could that be it?? I havent check the O2 sensor or replace it but from all the info giving can any of you guys come up with an answer of what is wrong with this vehicle? This is the one i beat with my carina and 7afe... That proves it isnt running properly.