Odd starting issue after hot

Finally got the Mustang out for the first time this year. She ran great. One odd thing that happened, and it's happened in the past, is this strenuous sounding startup after it's sat for a while when hot.

For instance, we cruised around for maybe 20 minutes at about 25-30 mph or so yesterday. Then we anchored for an hour while us and the kids cooled off in the water. Go to start her and it sounds like something's holding her back, she give this hum type of whine and turns over once. I try again and engine doesn't turn. Then one more time and she starts up and the alternator belt squeals for about 5 seconds.

It sounds like there is some kind of resistance going on. This only happens after she's been hot for a while. I even hear the hum when I shut down before this happens. It's really hard to describe the sound, will have to video it next weekend.

After we got back to the dock I shut her down for about 2 minutes and started up with no problems.

Engine runs cool the whole time, no other problems that I know of. Oil is fresh, etc.

Indeed, the checks that Peter prescribed.
Then timing. Slow cranking speed when hot suggest too much initial advance.

Pull the coil wire off and if it cranks normal, likely too much initial.

I had an issue similar with my 84 2001 ski nautique. My issue was the exhaust riser gaskets were bad and water would go back in the exhaust causing it not to want to start.