I have a 91 Chevy G 30 van–diesel (4L80E transmission)–3 speeds and overdrive. It’s not super-high mileage–100K miles. I Had some issues with it slipping out of overdrive after driving it for a few hours, then It will sometimes lock into limp mode (2nd gear only). Hitting bumps in the road make it slip for a half second, then it goes back into gear. Letting it cool off for a few hours seemed to reset it and make it work ok for awhile. A hard bump in the road makes it lock into limp mode sometimes…BUT the other day, it started in limp mode and a bump in the road reset it to working…WILD. Sometimes it works fine all day. It doesn’t slip when shifting–ever.
Anyway, I replaced the fluid, filter, and both shifting selanoids as an easy start. The fluid was clean and red and didn’t smell burned. Any suggestions??? I’d love to avoid going to a transmission specialist, but I fear that I may have to 
Thanks!!!
Stephen