I don't think it'll be a setting or outside interference - more likely a monitor fault coming on. Difficult to know what's the best course of action in these cases - probably wait until it gets really bad, then decide whether a repair or a new one is better.
Sounds like either a capacitor on its way out or a "dry" joint on the circuit board. As DieSse suggests, wait and see how it goes, it'll probably be as cheap to replace as repair, plus you get the warranty with a new monitor.
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