Why does my 1998 gmc truck backfire and stutter sometimes stall when driving

My friend the car backfires if its runing lean ( insufficient fuel ) or runing rich ( insufficient oxygen ) this mostly in carbourated cars would make incomplete compustion . in engine with fuel injuction there may be a leak in the intake valve that makes the fuel to burn in the exhaust pipe . so check for leak asap

I would suspect a burnt valve. An engine compression test will verify this.