@TonyC and others, I got round my similar problem with one of the suggestions in this link

Just thought that I would update this as since it got upgraded to the new firmware version last December it has not hung once. It has been left on tuned to BBC HD & other channels for extended periods (days) but has been working perfectly every time that I want to use it.

Nice to know that it was in fact a software problem & not hardware after all. Thank You Humax!

I sold my first HDR, it was one of the first in the UK when released. It always ran VERY hot (touch temp. must have been 50C plus)!
I sold that on ebay and bought another in Nov09. This one is MUCH cooler. So much so that I have disconnected the crappy noisy fan and it has never locked up (yet remains cool to touch).
Get the latest revision.. it is much more stable.

I've got one of the first HDR's as well from Nov 08. Mines never locked up, the case is cool to touch (in fact I would say it is cold even when left on 24 hours a day on live TV) and my fan is silent (in fact I can't hear it even in a quiet room - I only know when it is on because I can feel the air movement generated by it when I put my hand near the vent).