What Dave said. It's the starter. When you've run for a while it gets hot and this affects its ability to deliver. I have had some small issues but always started to date. Some people use a heat jacket to help with the problem also.
Check timeing... first
check, or duplicate ground second
run a short ground wire from one of the starter bolts to a bolt on the motor mount....to duplicate the ground... My std ford block to chassis ground was severly inadequate even on my stock block 302. you can buy this with both ends on it at advance auto for about 3-5$.
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