My S3 has just started to do this. I charge it overnight, wake up to find the alarm hasn't gone off, I've missed work, and I can only switch the phone back on after taking the battery out and putting it back in again. It's 100% charged when it starts up again.
It's happened 3 times in the last 2 weeks. I changed network a couple of months ago (from Vodafone UK to 3 UK) but can't work out if that would cause this to happen. After missing work yet again today I've uninstalled all recent apps. But is this a software problem or a hardware (overheating) problem?