I have the same issue. After 2-5 minutes I am logged out. It is not like a proper Firestorm crash and happens with the SL viewer as well. All was well until 2 days ago. Nothing in the setup has changed and I don't think it's overheating as the laptop is still cool (-ish). So it's either something in SL itself (is this possible if two viewers have the same problem) or my ISP. I had TalkTalk run a test on the line and they found nothing out of the ordinary.