I'm sure I will miss some important details, I will update as needed.

This is my work truck, I need it running well ASAP. As I am driving aprox 65-70 mph I will have light pressure on the gas. At random the dash gauges go nuts, the RPM plummets and "Service Engine Soon" light kicks on. I let off the throttle then it picks back up, I reapply but it just revs very high and is out of gear. I let off again and it will grab a gear and go back to normal. I have no idea what is going on, I am looking for tips on where to start.

I wasn't having any issues a few weeks ago. I took the truck into a shop to get the AC fixed and the mech told me he had to bypass something the head unit. After he did that all the gauges will go crazy randomly. I didn't immediately have these shutting down issues though. He told me he followed GM Bulletin #92-201-1b.

 

Sounds like an electrical problem to me. One problem on these older trucks (I have a 1996 Cheyenne C1500) is degradation of the grounds, and broken ground straps under the hood. Since it is likely a 4L60E or similar transmission when there is an electrical glitch, the electrically controlled solenoids in the transmission will probably get reset, and you will be out of gear.

I would double check ALL connectors under the hood, and look at the grounds closely as well.

 

I'll look into that today, thank you.

 

I blew a tire and had to get that taken care of first, but I had some time to dig into it today. I found a spot near the brake pedal that has dozens of wires running through the firewall. Some were in connectors and others were run in loom through it. I found that the large plastic connector where most of the wires from the cab run to the hood had two screws coming loose, I tightened them down and that seems to have solved the problem. I assume many of those ground through the connector. I took it on a test drive and everything seems to be working.

Im defentily going to keep poking around but hopefully I got it. Thank you for your help.

 

Sounds like you found the issue. Glad it was that simple!