Intermittent interference "could be" caused by
a computer speaker too close to the monitor?
And if turned of the picture will appear as normal
I have a speaker which causes a screen flicker if turned on it is caused by the transformer inside the speaker I could shield it by putting kitchen foil on a card and placing it betwixt monitor and speaker
In your case it sounds like a high wattage Item or something with a strong magnetic field nearby
Suggestions ...Fans, Heaters, Hifi speakers TV set or is the computer near a microwave? within a few feet or is it on the other side of the wall,Add Washing machines tumble dryers and electric cookers.

If any of the above seem to be close to your computer then resite your computer elsewhere today!

Worst case scenario..

Constant exposure to magnetic fields can effect your hard drive causing bad errors

Have you Had any BSODs yet or random fatal errors?

If so this is a clear indication that there is magnetic interference
Also ask the opinion of a TV repair guy if you can't find anything ,might be worth paying for an hours Labor? He may be able to spot what is causing this?

But do the things I have suggested first,

that will save you a lot of grief and money
