Engine bogging down

I have a 1993 chevy 1500 with 197,000 miles with 40,000 on the 4.3 v6 motor. My engine has always bogged down since ive owned the truck. The truck boggs down upon acceleration. Like it wants to stall then it takes off still feels like it has a miss. Ive changed the spark plugs, wires, cap and rotor, oxygen sensor, pcv valve, fuel filter, fuel pump, ive changed timing before to see if it makes a difference and ive checked the throttle position sensor, egr valve, fuel pressure, vacuum leaks and they have all checked out okay. It also always passes smog. Sometimes when it boggs down it backfires at the throttle body. Let me know if you think of anything I can check. Thanx