Archived from groups: comp.sys.ibm.pc.hardware.video (More info?
You may be loosing one of the colors.
The next time it happens I would try:
1. Slap the side of the case hard with your open hand.
If the colors come back properly, you probably have a bad
solder connection, usually on the CRT neck board.
2. Try bending the video cable, especially near the end where
it connects to the computer. If the colors come back, you
probably have an intermittant break in the video color wires.
You can purchase a new 15 pin HD connector at Radio Shack and
solder it to the wires, after cutting about 6" off the cable.
You might want to download a monitor color test program from the
web and see if actually one of the colors is missing.
Hope this helps...