Cause - Possible bad monitor, possible need of additional grounding
1. First and foremost, make sure the latest video drivers are installed.
2. Check to see if other monitors have the same effect when plugged into the system.
3. If the monitor is determined to be OK, switch the External Power Supply on the unit with another from the same model, and always be sure the unit is plugged into a surge protector.
4. An additional grounding plug (pictured in attachment below) may also be required; these can be found at most dollar and/or hardware stores.