I'm start facing this weird behavior today.
I am using the NUT .757 software, clean install, it was okay but since today my power button doesn't respond anymore. When i press the Volume up, the power menu comes out. Sometimes the Power button act well, but is just for few time.
Ps: I was in a pool taking pictures. The phone was totally closed, no water inside, but since then it start to act weird. Actually, sometimes the scren wont turn off, and if i press to turn off the phone, it reboots itself.