What bugs me is the randomness of the reboots. Sometimes more sometimes less often. For example my gf's phone didn't reboot for 4 days but today it did. I thought maybe because she doesn't have an alarm active on vacation that this somehow made a difference but no such luck.
If it really has something to do with battery/cpu temperature and downclocking the cpus when idle, this could be very problematic to fix, especially because of different brands of batteries and possibly other hardware in these mi4 phones.