Could be your O2 or even your ecu (or both)....sounds like how the car will run if you disconnect your eha and tune it manually...

Not sure if it will work for your 2.5, but to test my 2.3-16V we installed an ecu from a 2.6 (could not find a good used ecu for a 16V)...guess what..it ran fine...and as I had such a hard time finding a good used ecu for the 16V, I just left it in...been running with that ecu for more than 2 years now.

Some have claimed to be able to resolder the connections on an ecu, but when we took my faulty one apart, we could find no visible cracks....(under magnification)..that's when we decided to just try an ecu from a 2.6 to see what happened.

Before the ecu change, the poorly running motor managed to clog up the O2's very quickly...so the car would run like a raped ape for 10 minutes to 2 hours after replacing or repairing the O2's..then back to the same status. I replaced the O2 three times before we figured out that the ecu was causing the faults, and making the exchaust so rich it would blind the O2's....I was able to salvage the second O2, bench test it and clean it, so I didn't have to buy a third one.

Once we got the ecu done, we tuned it normally, and everything has been running fine since (until timing chain guide just failed).