Underneath the airbox bolted to the inner fender liner is a terminal stud for a power cable going from the batt to the Fuse Relay box in the cab. These studs corrode heavily and give you a high resistance connection. I bet a million bucks thats what your problem is. Pull the airbox completely out and you'll see the heavy gauge wire that runs through a 1" square plastic cap. Pull the cap off and see how nasty that connection is! Keep us posted.
Timmy