Reasons for lockup, video cranked too high, bios settings wrong, maybe a psu, multiplier/fsb cranked too high, etc. We would need more info on a random lockup. Is the board/video overclocked? Is there new hardware/software? etc....
I did not see anything about random reboots....
If it is in all games only, I would look for a video/video card conflict..
Lockups by themselves do not necessarily indicate a power supply problem or for that matter do random reboots. It is not that difficult to get the computer to do random reboots.
As for the SB causing this, I have read about many problems with these and compatability. I do not (and would not) have one of these due to this problem. There is a lot of information available regarding this on the net. I know that bios revisions on occasion have cured or aided in the cure of this.