1991 Ford E-Series Van timing?

Van backfires thru throttle body when I punch it from a dead stop. Only does it once and after I get going all seems fine. Have tuned up and no change have changed wires, com, ecm checked timing, seems to run better the faster you are driving. At 55mph seems unsteady 60 or above smothes out and runs great. Thought it might be fuel delivery so changed fuel pump and fuel filter still the same thing no change. No codes show up and engine light never comes on help! I used to be good working on vechiles untill they added all the stuff we dont need. Where do I look now Ford dealer will not work on this old of a van I need it for work so have been driving it as is.