GM-GMC: electricle vs. trany problem, chevy 3500, vaccuum

Question
First off thanks for taking time to do this I really appreciate it. So I have a automatic 1992 Chevy 3500 dually I use for work. It is doing the weirdest thing. I only use it once or twice a week. It starts up fine and runs great for about the first 20 min. of drive time. Next thing I know the check engine light comes on followed by  it drooping into 2nd gear. As you can imagine the rpms rev up really fast. I can't even take it out of second manually. Then the next time I try it does the same thing. I've heard both electrical and trany problems from different people I have asked. What do you think? and how do I fix it? I really need the truck so if I had to I would change out the trany. Thanks again for your help.

Answer
Pat ,

Ok , You say approx 20 minutes ? Is this all the time ! If so , something is heating up that is causing your problem !You might also have a modulator problem w/vaccuum problem .Its 92 , so the OBDII is out ! Electrical problem would even keep you from shifting into manual as well because the solenoid is keeping it in gear , You might want to check :

Actuator : Solenoid : Vaccuum



Let me know !



Dave G