I have an '89 trans am (305, TPI, 5-speed, 80k miles) All of a sudden after filling the gas tank my check engine light came on. Car ran fine for 15 miles or so then began to run very rough with loss of power. After putting it away for a few days I went to start it up and it ran fine so I reset the computer before getting any of the codes. Took it for a drive later that day and it started acting up again only after it got to operating temp. I thought it was just bad gas so I put about 300 miles or so on it but fresh gas didn't help. The check engine light has not come back on since I reset it. I replaced the 02 sensor thinking once it was running on an open loop it started acting up. Runs fine when cold but sounds very "flat" when it warms up. Doesn't stall and no problems starting. Sorry for the book, but I'm at a loss here. I don't trust taking it anywhere around here so any help would be appreciated!!

Thank You!