98 Blazer 4x4 4.3L Engine Fault: Code P0404 & P0420 Diagnosis and Solutions

I've completed extensive maintenance on my 1998 Blazer 4x4 with a 4.3L engine, including vacuum checks, new tune-up, air filters, intake manifold gaskets, and today replaced the upstream O2 sensors. Despite these efforts, I'm still experiencing persistent issues indicated by fault codes P0404 (Misfire Detected - Exhaust Gas Recirculation) and P0420 (Catalyst Efficiency Below Threshold). The engine maintains good vacuum pressure, which suggests the issue isn't directly related to the catalytic converter. However, during a 100-mile test drive, I noticed red exhaust pipes—this was alarming—and had to pull over after just 15 miles. This led me to suspect that the EGR valve may have stuck open, causing a lean burn condition and resulting in excessive exhaust temperatures. I've reset the codes at an auto parts store, but I'm hoping they don't reappear. If anyone has experience with similar issues on this model or knows of specific troubleshooting steps for P040 or P0420 on a 1998 Blazer 4x4, I'd greatly appreciate your input. Thanks in advance! Patrick