Spontaneous reset!!

[AuraEdge]

Ahhh!! is this ever weird!!
Ok..So im playing Half-life - Team fortress, my newest and greatest comp pasttime, and play through about 5 stages (long, long night)...When trying to load up the sixth stage, The comp resets! This has happened to me once before and I dont know why..Its always at the loading screens between stages. (No i didnt kick the reset button or anything like that...)
What kind of problems cause a spontaneous reset like this? I have a 300w power supply, BH6, P2 350 (not OC'ed) and a CL TNT (not OC'ed), and a 64mb PC66 stick @ 100.
Only thing out of spec is the PC66...could that be the problem? I dont know if memory failures can cause a reset tho..Id think it would be the power..but i have a 300w (Mobo, proc, 4 fans, TNT, SBL, CD, CDR, Floppy, 56k V.90..I still think I have some headway on the 300w)
So...any diagnositics for me?

[skywalker[TSG]]

ATX?? sometimes the ATX mobos reset them selves becsuse of the soft boot or what its called u know the thing that turns your PC off without u hiting the powerswitch this sometimes screws up and makes the comp restart

[MrEd]

BH-6? That could be the problem. I've got a BH-6 also, and I've experienced phantom reboots off and on but on very rare occasions. I'm not sure of the fix as of yet.