Ford Repair: ranger question, throttle position sensor, vent cap

Question
I have a 1991 ford ranger with a 2.9 liter automatic.
When I accelerate the engine bogs down and pops in air box
if I accelerate very slow it sometimes goes fine.And at higher speeds it sometimes will go great and sometimes losses power I have replaced plugs,plug wires,distributor cap,rotor,fuel filter twice ,air filter,air intake sensor, throttle position sensor and gas cap. the book said it takes pre vent cap not vented was on it I am at a loss your help would be very helpful thanks steve

Answer
What are the trouble codes?The gas cap is the last thing to look at.I understand you're frustration.Are there any fluids where they are not suppose to be?It sounds like you either have an intake gasket gone down.It is also possible that you have a worn out timing chain.I would start with the timing chain first.You might find it easy to do both at the same time.Another thing to look at would be the EGR valve.The check engine light should tell you.