|Borrow another external monitor to confirm the fault is within the monitor (or not).|
The symptom you describe is pointing to/suggestive of problem with monitor power supply; or a smoothing or decoupling condenser(capacitor) on the main voltage rail.
If you connect your monitor to another computer and wait the hour or two... that also may confirm the problem is within the monitor( or not).
When the fault appears, do not reboot the computer. Simply power down the monitor (remove mains power completely), and let the monitor cool down as it were. Then after a while power up the monitor again and see if the fault is still there (or not). If it isn't it points again to the problem being within the monitor. Leave the computer (booted up/logged in) on the whole time whilst the monitor is cooling down,,
Whether or not it's worth repairing, depends on the fault and how much it may cost?
message edited by trvlr