I read where you said that you still had the original, braided ground cable. Even though you cleaned the connections, if that is the original cable that would be my next, cheap place to look. I had the same problem you refer to on my '52 3100. Like yours, it is still running 6V, stock electrical system. I refuse to upgrade to 12V because I am stubborn. It still has the original wiring harness. Anyway, after rebuilding starter, cleaning connections, new battery, etc., I removed that braided, original ground strap. Seems that over time a LOT of those little wires that make up the braid had broken...many more of the internal ones (that I couldn't see) were no doubt broken too. I replaced that strap with a 0 or 1 gauge cable and that solved my problem.
