[doveman2]

Can you still ssh in after this lock up?
It certainly could be caused by overclock or power supply. Can you disable overclock and see if it still occurs, and double check voltage (when playing and recording).
Another possibility could be memory running out. Can you try running Milhouse's bcmstat.sh script and see if arm or gpu memory gets very low before crash (although typically xbmc just exits in this case, and restarts).

I don't think I can ssh in (pretty sure I tried this previously but didn't the last couple of days). Even if I have WinSCP connected before the lockup, it loses connection and can't reconnect once it occurs.

I've only got

arm_freq=900
over_voltage=2

at the moment, so nothing extreme but I'll try disabling it anyway and try checking the voltage and maybe using a different PSU and try Milhouse's script in case that reveals anything (although I still have the debug OSD showing and that indicates there's plenty of free RAM and I'm only running at 720P with 16-bit textures so I wouldn't have thought the GPU RAM would get filled).