It's all about ambient temp and relative humidity. High 90's in the shade and in the triple digits in the Oklahoma sun is going to heat up a motor fast the way we were running. The higher than average humidity didn't help.
I was running my buddy's RC8 in the Texas summer heat about a month ago, so conditions were the same as you're describing, or worse.
If your buggy has plenty of speed, do ya think it could simply be a little overgeared? Dropping down to a 14 from a 16 in my Losi 8ight made a 20dF difference in my motor and ESC temps.