I am not sure why my Core hardlocks every 6-8 hours with the DVB-C card installed, it might be the reception. The strange thing is that it never locks up before 6 hours, and never last longer than 8 hours... A reboot resets the "timer".
Now I just Dualboot the LivingRoom MD with XP, and use both DVB-C with HD channels and DVB-S + S2.
If anyone have any tips on how to solve or debug this, please tell...
If your system comes right back up after reboot and does not then crash quickly I think it is pretty unlikely to be heat, though checking using fans and opening the case is a an easy way to make sure it isn't.
I was having lockups because the USB driver in the kernel in LMCE 710 is known to have bugs in it. This was the USB hub built onto my DVB-T card (Dvico Fusion HDTV Dual Digital 4).
I eventually built a new kernel (220.127.116.11 I think- away from server now, but don't go to 2.6.28), compiled the latest V4L device drivers for my DVB-T card from the LinuxTV site , compiled the LIRC modules I needed, and it has been solid as a rock since then.
I did have another problem which would cause unreliable booting but that turned out to be a corrupted table that was crashing mysql during its "check" phase during reboot, and disconnecting LMCE and Mythtv processes randomly from Mysql.
What motherboard do you have? Also it might be worth a trawl through the mail archives from LinuxTV.org to see if any of your DVB cards have known problems. Updating your DVB drivers using the instructions at http://www.linuxtv.org/wiki/index.php/How_to_build_from_Mercurial
is not too hard.