Is the plug loose into the Vid Card?
Could it be the Vidcard..Try the monitor on a known good system..
I guess those should be checked first.. lol
Sometimes I just automatically think that those simple things have already been checked...

Faulty solder connection on the CRT Card (specificly from the Data cable..as most pc's the vid cable goes directly to the CRT board..)
Does that mean that monitors should not be carried by those cords? LoL
I saw a guy moving some office equipment the other day who came walking out the door with two monitors. He was holding them each by the cords...