I just had a new catalytic converter (a universal one) installed on my '90 4cyl Trooper. On the way home from the shop, after driving thru town about 7 miles, the engine started stumbling and hesitating, totally losing power. I was able to limp it home and later that day I purchased a new O2 sensor.

I installed the new O2 sensor and disconnected the battery to reset the ECM, hoping that this would fix the situation. It did not. The Trooper runs great until it gets nice and warmed up, then it starts having the stumbling and hesitating problem. I replaced the thermostat and temperature sending unit a few weeks ago, so I know that the Trooper is not overheating, and it had been running fine until my trip home from having the catalytic installed. I also ran the self-diagnostic on the ECM and there were no error messages.

I am thinking there is something wrong with the new catalytic converter, am I right?

Any advice would be appreciated greatly, I have gone thru my shop manual and nothing I have done seems to work :/