It is possible that the engine is tightening up to the point the starter can't crank it fast enough when it's hot, but that's a remote possibility. What is the battery terminal voltage during hot cranking? Without some measurements, we're all doing what we called a SWAG in the Air Force.
Scientific
Wild
Azz
Guess
Back when Sun made the best test equipment in the automotive business, every piece of their equipment had the same motto on it: "We test, not guess!" Sun went belly-up about 30 years ago and Snap-On bought what was left of the company, but those principles still apply. If the voltage is dropping way down with a hot engine, you might have internal engine problems that neither voltage regulators, batteries, starters, nor cables can fix.
Jerry
