Interesting report. I have been flying in 90 degree temperatures but have not let it sit idle in the sun long enough for significant sun loading. Once airborne, I think that there should be enough air flow getting thru the canopy to keep the IC cool enough. I will try to test both of my DFs to see if it can be repeated here. As I'm sure you know, a heatsink (even a small one) with a thermal conductive adhesive, could significantly reduce the chip temperature. Take a look at the bottom side of the PWB and you'll see they use a little washer as a heatsink on one of the semiconductors.

Mike