half crashes no access minidump file no event record

I used to run XP on my machine, stable as "solid rock". Than I got Vista beta 64b, and all this Beta stuff. And start to have those "half crashes": my monitor suddenly gets black, no video. Nothing to do. The machine looks like normal, i.e., keeps accessing disk and if I touch the power off for a secund, the shuting down goes normaly, everything is saved and closed and machine goes off normaly. Afeter reboot, no clue on events logs.
So I opened the box for a memory upgrade, and find out the chipset fan very lasy. Almost stopped and video card heatsink with a lot of dust.
To keep things short: after a cleanup and small repair, the machine was "rocking" again. Vista puts a lot more heater in the box than XP. Keep this on mind.
BTW - MB is asus A8N-SLI Deluxe, 1 year old, chipset fan replaced!
Rgds4all
fglima