Hard Cranking When Hot

As I read it, the battery is 10 yrs old.?? The avg life expectancy of a lead/acid battery is 5 - 7 yrs, so that's be the first thing to check. It could read 13+ volts, but have no "capacity" to crank.

If I hadn't read the 10yr thing, My first thought was strongly recommend trying another starter. For the full sized starters, a lotta rebuilders just dissmantle and toss a buncha mopar starter parts in a cleaner, and re-assemble generically into one part #. So that replacement starter may have had a less than "440" suitable armature.

I'd try a different starter regardless, hopefully suitably assembled.

Been where you are, a few times, that was the cure. ( ^#@$&*^) lol

hope it helps. cheers