Since it started after doing the work, specifically the exhaust, are you sure there isn't a disconnected or damaged O2 sensor? Also, have you checked for diagnostic codes?
The things you mentioned could all cause what was described, but since it started after the work, I suspect something happened during the procedure.
Do me a favor. Check to see if there are any diagnostic trouble codes. This has an OBD1 system. You don't need a scanner for it. Follow the directions in this link and let me know what you find:
Let me know if there are any codes. Also, let me know if there are any strange smells or sounds. I'm not real familiar with the flowmasters you installed, but there could be an airflow issue if installed in the wrong direction. I know the Flowmasters are chambered, so there would be a flow restriction if backward. It's just a thought.
Let me know.
Thursday, July 1st, 2021 AT 9:49 PM