P0420 Fault on 1998 Chevrolet Lumina 3.1L Automatic - Loss of Drive Performance at High RPM

I own a 1998 Chevrolet Lumina with a 3.1L engine, currently at 86,000 km. After driving on the highway for about 10 minutes under heavy load, the CHECK ENGINE light activates with fault code P0420. Following this, the vehicle loses driveability and stalls at around 2,000 RPM. I have reset the onboard computer, and while the CHECK ENGINE light turns off temporarily, the loss of performance persists—after restarting, the car still fails to respond properly beyond 2,000 RPM. Previously, I replaced the spark plugs (P0174 code) six months ago. At idle (both parked and engine running), the accelerator pedal provides a smooth, linear increase in engine RPM up to 5,000 RPM. Recently, after approximately 20 km of driving, the CHECK ENGINE light has not reappeared. However, the performance issue remains unresolved. I am seeking advice on how to diagnose and fix this P0420-related problem. Any insights or recommendations for this specific 1998 Chevrolet Lumina 3.1L automatic model would be greatly appreciated. Alexey, Kiev, Ukraine