What temperature are people seeing who are getting full hash rate and minimal rejects? That should be the target. You may not ever see 60C with full, valid hashrate unless you put it in a freezer.
What is your ambient temperature? Have you tried putting it in a cooler place? I assume you’re not willing to crack it open and make some cooling mods?
I suspect what is happening is that there is a hard limit of ~100C in the chip, as there should be, and it is throttling to keep the temperature just below that. It’s a little weird that it seems to surface as rejects instead of just a reduced hashrate, but not entirely surprising given the circumstances. 
Edit: this post says there is a configuration setting for changing the fan speed, do you have this option?