Sounds like a hardware problem to me - like bad/dirty connections,
overheating, or monitor going bad. Most likely overheating display card
or monitor with a problem, in my mind.
Make sure things are clean and well ventilated in the CPU case, that
there is plenty of airspace around the CPU case and the monitor, cards
and RAM modules firmly seated in slots, all fans working - including the
fan on the video card if there is one, display cable secure, etc. then
swap out the monitor if it persists.
Steve