2002 Pontiac Grand Prix OBD2 Code P0404: How to Fix Poor Power on Hills and Lean Running Symptoms

My 2002 Pontiac Grand Prix is experiencing severe loss of power when climbing hills or accelerating. I've already replaced the EGR valve, ignition control module, muffler, air filter, and fuel filter. Fuel pressure remains stable at over 50 psi with less than 3 psi drop from the cat converter during compression testing (160-180 psi). No vacuum leaks were detected, and I've installed new upper plenum and intake gaskets. The vehicle has 110,000 miles and recently had a tune-up including plugs and wires. I've also swapped out the MAF sensor with no improvement. The engine appears to be running lean—plugs show white and chalky deposits, and there's visible soot in the exhaust manifold and pipe. OBD2 code P0404 is present. Despite these efforts, performance remains poor. Any insights or troubleshooting steps would be greatly appreciated.