So this happened again. I was browsing Facebook with Dolphin browser, and suddenly the phone did not respond any more. I restarted the phone, and it begun heating up while I was doing absolutely nothing with it, and the battery was draining very fast, say 5-7 percent per hour in Stamina sleep. No calling issues this time though.
Rebooted a couple of times without any help. After a few of hours the phone fixed itself again and behaves normally now. The heating and draining issues have gone.
This is not as serious as constant issues which have been reported here, but it's odd they keep coming and going for no obvious reason.
I have a feeling this thing won't survive through the guarantee period. One and a half years to go.
