1999 Sonoma Automatic Gremlins

[resonance530]

I joined this forum in an attempt to find the truth. When I purchased a 1999 Sonoma with an auto tranny and a four banger, my goals were reliability and economy. Everything was perfectly maintained by the previous owner, and checked out with flying colors. Then, a couple months after the purchase, on the freeway, the service soon light came on, and it felt like the lockup on the torque converter let go, and then a few miles later, the tranny started slipping all together.

 

I limped the truck home but by the time I was in the driveway it stunk like cooked fluid. I pulled the pan, replaced the filter (no noticable shavings or debris), and filled her up wth fresh fluid. When I restarted the truck, the light was still on, and the tranny still slipped between shifts and would not lock up in overdrive. I took it to a reputable local tranny shop, and they determined that the entire trans was shot. That is not in my budget, so I told the guy I would limp it back home and park it until I had the money. Here is where it gets weird:

 

The next day, when I started the truck to move it further back into the yard, the light wasn't on. I cruised around the neighborhood, and the tranny runs perfectly. I got it out on the freeway, and it was perfect as well, settling into overdrive and cruising at about 2000 rpms. I swapped the fluid out again to get as much burnt yuck out as possible, and it has run great ever since. This would all be great news if it weren't for the fact that I bought this truck for my girlfriend, and she heads out into the mountains in the middle of nowhere to mine crystals and gems all the time. The idea of her getting stranded out there is not good. SO, after all my babbling, does anyone know what's going on? It seems llike an electrical problem, not a mechanical one. It comes and goes instantly, and I've never known an auto tranny to do what this one did without dropping altogether. Help me please.