Are you sure about the model? I don't see a "Dimension 8600" listed.
- Scanned thoroughly for malware recently?
- Can you test this monitor on another PC, or a different monitor on this PC?
- Check video cable connections at both ends
- Reseat the video card in its slot, assuming you're using an add-in card, not onboard graphics
- Uninstall the video card in Device Manager and install the latest compatible version of the video driver
- Try a different video cable
- Run chkdsk /f on the hard drive