Nice Tim you quote a day that you all did not race on.
The other days you all did race in September the temps were 79 81 and 75. OK maybe not 20 degrees. But some days in July and August were near 100.
All I am saying is you have to factor in the outdoor temp more than you are and not dismiss it and say you found the problem. The only way to prove your theory is have a constant outdoor or indoor temp.
I would like to know what the indoor results will be? This would be harder to do since it seems indoor 10.5 races both rubber and foam do not have a problem with this over heating issue. As much as your local racers. Maybe it is the indoor temps are normally lower and the humidity is lower. The tracks are smaller so the gearing is more conservative. Could also be the wrong gear choices to begin with on the outdoor track.
Even indoor guys are cooling their motors and do not have as big of an issue in frying motors.