you may have to recap the digital board (the board where the VGA signal goes in and then out to the panel) and/or have an issue with a regulator on the digital board. often the caps start to go and it stresses the regulator. noise in the power caused by failing caps can cause all kinds of weird issues.
if you use a good voltmeter and measure the 5 volt and 3.3 volt regulators and they measure good, test with the meter on AC...this will give you an indication of ripple in the DC voltage. anything over 20mV AC should be cause for concern.
These monitors are getting quite old now, and the caps are going to start failing. I recap every electrolytic on the boards when i start having issues with them.