Recently, I noticed that my phone shuts off while in sleep mode. This just happens when I don't check it for a while and the display is turned off. When I do go to check my phone, it's unresponsive, even if I hold down the power button. So far, I've had to remove the batter every time this has happened in order to reboot it.

I just don't get what the issue is. I'm probably missing phone calls because I'm not even aware that my phone is turning off. I feel like after the ICS update my phone has had issues. I already performed a factory reset last week since I was having a problem with the notifications not ringing and vibrating, and now, this is happening on top of that. 

If anyone has any suggestions, that would be greatly appreciated.

I have a Samsung Galaxy S2 i777.

Oh, and another thing, I noticed that my batter is at 95-100% before it shuts off by itself, and once it boots up again it's down to 85-89%. I also feel like it might be an issue with overheating: I noticed that my phone feels hot after I've been using it for a while, like texting or using the internet, and then I set it down to do other stuff when I notice that it's turned off. My phone usually feels really hot too, and the case I have for my phone probably does not help either.