LOGOFF Forces Shutdown & Ree-boot

I have a Gateway DX4200-09, AMD Quad-core,
4GB and running Vista Home Premium 64bit. When I try to logoff, it
starts to properly, but then the screen goes blank and my monitor
informs me that there is no connection with the system (but the system
is still powered on). No matter how long I wait, up to 1 hour, I have to
power-off the system to re-boot. During the boot, it tells me that the
previous action did not complete properly.

This problem doesn't happen every time I try to logoff, but 95% it
does.

Any ideas?????


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lonach