|Hmm, could be a tricky one to sort out.|
You said it only happens when the computer is idle and the monitor goes to sleep.
You can play games for hours without any problems?
1) A bad screen saver can cause chaos.
- Change or disable your screen saver & see if that helps.
2) A device or it's driver may not be letting the computer sleep and resume properly.
- Try disconnecting ALL non-essential devices (especially USB devices) and test.
3) The video card or video driver is causing a similar sleep issue.
- Remove and re-install your video drivers. If you can: test with a "known-good" video card.
4) Disable Sleep & Hibernation in Power Management & see if that helps.
5) Your BIOS' power saving or ACPI settings have changed since you installed Windows.
- Change the related BIOS/CMOS settings back to their defaults (if possible).