catalytic converter?

My daughters 93 Cutlass Supreme 3.1L. will start and run perfect after sitting overnite. If shut off and restarted after it is warmed up it will bog down and lose power, but sometimes it will kick back in and run perfect. I have shot the catalytic converter with an infrared thermometer and it was 120 degrees F different from the front to the back. I have installed new plugs, wires, air & fuel filters. Any suggestions are greatly appreciated.

Good work on the cat, check fuel pressure when this happens, 35 plus is normal.