My first guess is timing to advanced.
Are you running a performance cam and headers?
Sometimes the headers will cause the starter to overheat.
My cousin had a 396 that did that. He added a ford starter solenoid to the electrical system and it seem to help. Boy it pi$$ed him off to think a ford part fixed his chevy.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"