Thank you all for your replies. If the issue is heat related I believe I will have a limited window to troubleshoot this since the 10 minutes or so of it not able to crank only existed after 60 minutes of 60mph driving (arriving at 1st yard sale.) Thinking it may be battery issue I hooked battery jumper to car and it started but I think it was more to do with it cooling down because I left car running for the next hour or two while visiting other yard sales and stopped by auto store for free alternator/battery check and everything seemed good. I turned car off for the battery check and only got click without crank again. Applied battery jumper and no change. After battery jumper was removed it started and this was only a minute or two while off and not able to crank. This was also after driving slow speeds in neighborhoods and much idling. Also on hour drive home same issue as previous hour drive, waiting about 10 or so minutes and starts instantly. This is what caused me to assume that high speed/temps (90-100c) caused it to take longer to cool while the same amount of time but at lower speeds/temps (80c) only took a tenth of the time to cool. I don't know if this is a new issue or its because outside temps are in 80s & 90s.
What made me first think it could be overheating of starter was reading this even though not Benz related..

"It's not unusual for a starter to "seize" at high temperatures if the internal clearances are excessive. You may ameliorate the problem (temporarily, usually) by surrounding the starter with high-temperature insulation. It that doesn't help, replace the starter."

I don't believe I will be driving again till next Saturday so updates will be delayed. I may just let the car idle till I get to a yard sale where it has cooled enough to expect only a minute or two cool down. Also I'm thinking a heat shield/exhaust wrap couldn't hurt. I will try to report back on voltages in the various states described when issues repeats and report back, as well as what jumping at junction box yields, and how much current it draws. Thanks everybody!