|If this is an external monitor....|
Try the monitor with another computer.
If the monitor works fine with the other computer, there's something wrong with the subject computer.
Some mboards develop this problem - electrolytic capacitors were installed on them that were not properly made, and they fail eventually - the mboard manufacturer didn't know they were improperly made at the time the mboard was made.
Open up your case and examine the mboard to see if you have bad capacitors, and/or other findable signs of mboard damage .
What to look for, mboard symptoms, example pictures:
Home page that site
- what the problem is caused by
- he says there are STILL bad capacitors on more recent mboards.
Pictures of blown capacitors, other components, power supplies, fried Athlon cpus, etc.:
If the symptoms are the same with another computer, there is something wrong with the monitor.
E.g. a friend of mine has an older LG LCD monitor that had symptoms like yours for a while until it produced no video at all.
I opened it up and found a failed electrolytic capacitor on one of the circuit boards.
I replaced that capacitor, and the only other one that was made by the same brand that was a similar larger size, and the monitor has been working fine ever since.
What to look for, example pictures: