I have had reboot loops exactly twice, both times when I changed the battery to a spare that I alternate. Each time the battery I was swapping to was at about 50 to 60% charge. It would keep on rebooting, whether on battery power, or when it was plugged in to mains power recharging.
A while back I bought a couple of batteries and a charger from Hong Kong. I find now, that if I charge the battery I am intending to swap to for a few hours, then swap, that I have no problems, so I would say my reboots are tied in to battery level.
Also, it didn't matter whether I was swapping to the Hong Kong battery, or the Nokia battery......if the % charge in the battery was less than maximum, it rebooted.