What Ala_ said: My battery ground cable lug at the ground end was damn near eaten away with corrosion when I got the car three years ago.
AutoZone had a generic cable that was only 2 inches longer and fit perfectly for something like $9.95.

And the little box next to the battery with three bolt in fuses - check the condition and tightness of the 200 amp fuse in there.

And *sometimes* it is hard to describe what a car is doing, exactly, so I'd suggest you have the RCM resoldered completely.