When transmission is put into drive/reverse it shuts off

My truck starts up rough and smooths out a bit after warming up. Once I put it into drive/reverse it shuts off.I can keep it running if I continue to apply pressure to the gas and brake simultaneously, but shuts off very quickly once the engine decreases to idle.

Here are the list of things I have done: cleaned and resized spark plugs, check spark plug cables/coils/pressure, replaced distributor cap, replaced O2 sensor, changed out the fuel filter, tested fuel pump, timing is good, checked voltage output of TPS, checked for vacuum leaks.

Computer codes read normal operation, no check engine light.

Any input would be appreciated.