I personally wouldn't have my 6 volt starter converted to 12 volt. The beauty of the 6 volt starter hooked up to 12 volts, in my opinion, is the faster engine speed during start up. Sure, you can't get away with extremely long cranking periods with it, but it turns so fast, you don't ever need to. I'd say that if all of your wires are in the correct places and clean, your starter has internal issues and needs to be rebuilt.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission