Sounds like a weak battery, first guess. Did he rebuild it...maybe high compression. You might want to convert to a PMGR starter. Ford used them in the mid 90s, and they should have an easier time turning your engine over when it's hot. You can also pick up some high performance PMGR starters, but the OEM should be an improvement. The 95 SN95 was the last year of the old pushrod 5.0 on the stang, so any O'rielly auto or Auto Zone should list it....or rock auto, etc. Around $50, these days.