Backfires under load and shuts off

When engine gets under strain like going up hill or in too high of a gear sometimes it will backfire. even busted the muffler but then it will shut off until the ignition switch is cycled off then back on. and if the truck is stalled on takeoff or if the engine shuts off then it won't start back without cycling the switch. It may do it once and then be fine or sometimes it will do it repeatedly after it happens the first time while driving.