I have a 98 gmc 3500 6.5 just replaced the injection pump because the truck would start run for a bit then rev full throttle on its own after 5 min running time then idle back down and then it finally when driving would just idle and when you hit the throttle the rpms would go up to 1500 then down to idle basically undriveable it looked like the truck had hydraulics brought I replaced the pmd with I think was a good one now I just replaced the injection pump and still has no throttle just goes up and down when I override the shut off solenoid with a computer it revs fine then take it off computer it goes back to up and down can someone please help is it a computer problem or pmd thanks

 

Sounds more like an electronic throttle problem to me.
Are all grounds clean and secure as well?
Would help if current trouble codes were posted.

 

its not throwing any codes its at my mechanics now I am going there tomorrow to check it out check all grounds and wires ill post tomorrow afternoon thanks