Ok strange problem, I didn't reinstall Windows 7 after swapping the motherboard and I'm wondering if it is to blame for this problem. It keeps rebooting randomly whilst idle or under low load such as browsing. Playing the UT3 demo on max settings or running the Prime95 torture test don't do anything to it, it runs flawless which makes me think it's not the temps.
Update: It's been rebooting again, I'm running Prime 95 on one core and it seems to be preventing it from restarting randomly whilst using the computer.
What kind of temps are you getting at the moment? I'm looking into a build much the same as yours.
This seems to be a common problem on the NVidia 9300 motherboards. A recent thread http://forums.overclockers.co.uk/showthread.php?t=18037025 was discussing a similar situation on a Zotac 9300 ITX. As a rule, it proves sufficient to re-sit the heatsink using a thin layer of high quality thermal paste.Well the problem has been solved now, it was the Northbridge chipset that wes hitting 96 degrees causing the reboots. It's mainly due to the built in GeForce 9300 and crappy passive heatsink. Running Prime95 or a game stopped the crashing as it caused the CPU fan to spin up, creating more airflow over the northbridge. The machine runs hotter than it would in an ATX case as the airflow is more restricted.
As a rule, it proves sufficient to re-sit the heatsink using a thin layer of high quality thermal paste.