I looked up some helpful stuff on over-clocking computers and used the suggested BIOS option settings to compare my own "auto/default" set up. It is quite enlightening. My BIOS auto-detects the wrong settings for my DRAM.
My Corsair ram modules are labelled 1600MHz, 8-8-8-24 1.6 volts. The BIOS insists on setting them to 1066MHz, 8-8-8-20 and sets less than 1.5 volts. Clearly the "20" can't be right. A visit to the data sheet gave further info, at 1.5 volts expect 9-9-9-24 and 1333MHz but at 1.65 volts expect 8-8-8-24 and 1600MHz.
After a reboot when playing videos I went into the bios and it reported "reboot caused by over-clocking or voltage errors", and went on to say some settings my be different to those displayed on the screen! It offered a "safe settings" option which set 8-8-8-20 for the ram and set the CPU to 2GHz. Not what you really want when the ram and cpu are both specced to go much faster but it did stop the crashing/re-booting for a day or two.
I note that there is a couple of later bios revisions available, the reasons for the up issue do not mention ram or cpu timing but do say there is an error in the QPI speed. Time for a re-flash, I think.
Rans6andrew.