My FP2 runs fine but it was rebooting every now and then. I noticed that it was only when it was put under some mechanical stress (also when twisting it slightly) or small shocks (putting it on a hard surface table) and therefore apparently unrelated to the reboot issues reported elsewhere. Also, it did not reboot when it was just sitting on a table over night. It also did not want to boot without the cover. After playing around for a bit I found that the battery was a bit too loose such that it was apparently loosing contact to the pins on top every now and then triggering the reboots. Now I carefully squeezed some thin cardboard in between the battery and the FP2’s plastic frame on the site opposite the contacts such that the battery is pushed against the pins on the other side with a bit more force. So far it seems to fix the issue. Slightly twisting the phone does at least not trigger a reboot.