1979 D300 short?

you running a gel cell battery? i had THREE AGM batterys crap out on me similar to what you have going on. they would lose a cell whenever they felt like it. i could literally hook up my volt meter to the posts, car off, read 10.1, hit the key, watch it tank to 8.5ish volts, fire up, (because i have MSD ignition in my rigs and it works to approx 7 volts) and then watch the voltage jump to 13.2-14.4, shut the rig off, and then read 12.4 volts, and be happy as a clam for days. it was the wierdest damn thing i ever saw. and all three would do this intermittently. does your truck still have the factory ammeter? ive never heard of one crapping out, then magically functioning again but i guess anythings possible.....