It happens even on low clocks and with a large box fan blowing very cool air at the blades.  I don't think it's heat based.  In fact, after further testing, they still reboot at the same time even when connected to different proxies/pools.

It's just not likely that two blades with overkill cooling are overheating & locking up every ASIC chip at the exact same time.  it's next to impossible, really.  Furthermore, if it were heat based, I'm sure i'd get much less than an hour out of them after the reboot.