1990 Chevy Suburban inconsitant sluggish accellaration and

My 1990 Suburban inconsistantly dogs during accelaration and the engine revs while holding speeds around 45 mph. The problem only seems to be present at normal operating temps, not while cold. I have a TB spacer, Air raid intake, and custom dual exhaust with flowmaster 40s, 2.25" pipes, and high flow cats. The timing, O2 sensor, EGR, and PCV are fine. There are no vacuum leaks I can detect. I pulled the tb spacer and intake off and went back to stock, problems persisted. Is it possible there is not enough back pressure in the exhaust system? It threw a code about exhaust being too lean, that is when I changed the O2 sensor. Any help will be greatly appreciated.