I had the same problem and put a lot of money into replacing things. My speedometer would jump and shifting would be crazy. It worsened and it started backfiring and missing, running hot and stalling out. Sometimes it would run fine. I replaced the distributor twice, burned through 3 coils, replaced the battery and alternator, all vaccum lines, most sensors, new throttle body, new computer and chip, and the drac. No luck, same problems. Finally a friend reached up to wiggle the wiring harness the runs the the front of the intake manifold and the truck shut off. It was the ground wire running from the harness to the front of the intake manifold. There is an alternator support bracket that mounts to the manifold and it had slowly backed out. The ground is attached to the same bolt. Everything runs perfect now. Check your grounds! It may save you hundreds of dollars. If you want to see how my truck was behaving, search YouTube for "1995 gmc sierra acting odd." It is under my youtube username sbdblyss. Good luck!