You might want to try the following:
Go to Start -> Control Panel -> System.
Click on the Advanced page.
Click on the Settings button in the Startup and Recovery
section.
On the page that pops up, remove the check mark next to
"Automatically Restart".
Click OK twice.
Hopefully, the next time this problem arises you'll be
presented with a blue screen with some useful information that
you can research on this web site:
http://aumha.org/win5/kbestop.htm
Also take a look in Event Viewer to see if there are any
entries that apply to this problem. You can access Event Viewer
through Control Panel -> Adminstrative Tools.
The few times I've had to deal with problems related to standby
it's usually been solved by updating the driver for the display
adapter. You might want to make sure that you're using the
latest one. Check the manufacturer's web site.
Good luck
Nepatsfan