I have a 1951 Farmall Super A that starts and runs great for about 30 minutes and then starts to mis for a minute or 2 and dies. Won't restart until it's cooled down for a few hours. Points and condenser are new, points gapped at 0.025". Was running on 12V battery with gen belt removed. Replaced 12V battery with a new 6V Group 1 from TSC. Thought that maybe the 12V battery had something do with it but still happens. I had the valve cover off to replace the gasket and while I was in there checked valve lash and cold everything was 0.018-0.022". Cold, dry engine compression is 90+ psi in all cylinders. Engine cranks fine when it dies but just won't start and run. I'm stumped. Any suggestions?