I have a 1994 Ford f-250 4x4 that is the money pit!! I have pretty much the exact same issue that I am working on. I have done some research and looked over my vehicle and can say this. Start out with checking all you're ground wires. (Engine compartment to fire wall, ground strap to frame)
The ground wires in my truck are in very poor condition. (rusted, fraid) replace the straps after cleaning the area where they are mounted. It wouldn't hurt to even add a few straps.
I have noticed that when I am running the vehicle, the battery guage on dash moves downward at idle, then upward with accelleration.
Let me know what you come up with. If it is just bad grounding, that is a cheap fix!!