No, it doesn't appear a time issue, more what I do triggers it. And its repeatable, that is I can create the problem just by running a scan using windows defender (and a couple of other things). Although it does happen occasionally at other times as well (possibly a background process such as indexing accessing whatevers the problem).
I've just tried turning the PC off, cleaned out the vents as best I could then turned it back on. After using it for 20 mins or so the temperature was 59C, does that sound better?
I then tried running windows defender and it froze the PC again at about the same point. The object it said it was scanning on the screen at the time was part of the registry. I've run the registry cleaner thats part of CCleaner and that found a couple of errors with unused file extensions but nothing that matched what was on screen when the computer froze (which was a HKCU internet settings object).
Is it possible something in the registry is corrupted? Is there any good registry scanners I could use? Or is there anything else I could try?