Am I right in guessing this is a CRT (Picture Tube) type monitor, not an LCD?
If it is, sounds like you're describing a fault called "blooming", which is caused by either the High Tension voltage to the CRT falling, or the monitor's PSU voltage rail rising, either way the monitor's protection is triggered by the faulty operation. You can confirm that the fault is internal to the monitor by running it on another computer system and checking that the condition still occurs.
If this is the fault you're experiencing, then the monitor will sooner or later fail completely. It may be worth repairing (now, not if it fails completely) depending on the age and quality of the unit, but more likely it could cost more than can be justified, considering the modest cost of reasonable LCD monitors these days.
Edited by Platypus, 28 December 2008 - 04:14 AM.