2001 Oldsmobile Alero P0420 Code: Persistent Hesitation After Multiple Repairs

I've been struggling with a recurring P0420 code on my 2001 Oldsmobile Alero for several months. The vehicle initially hesitates during acceleration, and after researching common causes, I replaced the downstream O2 sensor on Bank 1. Later, my catalytic converter failed, so I had the resonator, catalytic converter, and coil cover replaced. This fixed the issue temporarily, but hesitation returned shortly afterward—now with idle speeds fluctuating between 500 and 600 RPM. I then replaced the coolant temperature sensor, which improved performance for a while before the problem returned again. Most recently, I replaced the Bank 1 ignition coil (which appeared burnt to touch), and that was only three weeks ago. Yesterday, the P0420 code reappeared along with hesitation. At this point, I've changed five major components: downstream O2 sensor, resonator, catalytic converter, coolant temperature sensor, and Bank 1 ignition coil. I'm now out of budget for more parts and am completely stuck. Any insights or solutions would be greatly appreciated—this issue is becoming both frustrating and costly to resolve.