If the problem was drag related, ot would tend to get hot at high speed. That's not generally the issue. Drag increases as the square of speed, so slowing down would cool it off. That's not happening.

As for a cooler thermostat, it's not going to change the max temp, just the lower limit. I'll know later this afternoon if the thermostat change has made a significant difference. If not, I'm going to start playing with one of these thermostats to see if I can change this.