Hi, check monitor cable and connections for damage or loose fit.
If the problems persists, it may indicate a problem with the graphic card/chip or the monitor. I suggest, if possible, to use a substitute monitor to confirm or eliminate the monitor as the cause.
If the monitor is ruled out as the problem, then it may be a problem with the graphic card/onboard chip.
If a card, then remove, clean contacts and reseat into the slot. Ensure the card is clean and cooling fan, if fitted, is working correctly. If a high powered graphic card, ensure any additional power supply cables are fitted correctly.If problem persists, then remove and reinstall graphic drivers.
If an onboard chip, ensure the case has good ventilation and all case fans are functioning.
If the problem still persists, then it may indicate a defective graphic card/ motherboard graphic chip.