Doug, a friend of mine experienced the same thing with his 12v system as you did. His experience was in his roadrunner and his dart. 12v batteries, 2/0 cable, etc etc. We did everything possible to the 12v system to see what its problem was, and we found nothing. He switched to 16v like I had been on him for years to do. No more issues. His dart, same story with the 12v battery. I lent him my spare 16v battery, no more issues. His dart has a 511 low compression street engine in it. Why is it that some people have such good success with 12v batteries and others do not? I wish I knew.