Hello, and thanks for using FixYa.
The problem is most likely the powersupply, a few capacitors are most likely bulky - faulty, it is easy to spot them their sufrace is not flat and they do not work as they should anymore. They are generally 10Volts to 25Volts with uf ratings going from 100 to 1500. Replace the caps with new, matching value capacitors.
Picture of a GOOD capacitor. a BAD capacitors surface will be bent, bulky, or it might even be blown or leaky. But since your monitor works they are most likely just bulky.
Those capacitors need to be replaced to get the monitor to work properly, another way to solve the problem would be to replace the whole powersupply.
As long as this is the only fault your monitor has, it will be solved by what I've posted.
Please let me know if you require further assistance or if you have any questions.
Thanks,
Ekse