This happens usually when the battery gets low, around 50% and lower and im playing a game, not a very heavy running game, for example angry birds or jetpack joyride, but it also occurs when i use twitter or facebook at times as well. It also just occurred while i was just checking twitter and instagram while it was charging and it felt very warm. I have had this phone for over 2 years now, and i know im in for an upgrade, but is this normal? Im trying my best to hold off on the next version of iphone, but i honestly dont know if i can. Other problems i have, is the slowness of the device running on the most recent software, crashing, and freezing. My lock button is stuck so i cant reset my phone and sometimes notification center doesnt even pull down. Would a full system restore solve some of these problems? Would you guys upgrade now or wait? Just curious.