Samsung Galaxy S2, Battery Problem

I have a problem with my phone. The phone randomly emptying the battery (it takes approx 4-6 hours from fully charged). No heavy programs is in use when this occurs. In the beginning I wanted to use the phone as wake-up clock ...but I couldnt trust it for the battery sometimes works whole night and somtimes not. I can feel the phone is getting warm next to the camera when the battery-emptying starts. Any ideas?
I have tryed factory reset, upgraded to latest firmware, uninstalled unused programs etc.. but its not fixed it. But the upgrading of firmware made it better, so now its not happens so often ( 2-3 times pr week) Tip?