1992 Isuzu Rodeo w/ 3.1L Engine Manual 5 speed tranny

I drive the car and when on low side of gear it will backfire thru throttle body and loss of engine power. Replaced: EGR Valve, EGR Solenoid, Dist.Cap, Rotor, Cables, Plugs, TPS Sensor, Fuel Filter, Oil, Oil Filter, checked for vacuum leaks and found non, Engine power is not loss when not in gear, but backfires the same off gear after engine warms up, I change all that and still does the same thing, mostly when warm. I am in Florida (warm/hot weather). Going to replace PCV Valve today.