Before knowing the reasons, just check the follwing.
Recently did you move any electrical devices, just next or close to your monitor? Like . . .
- Printer power supply transformer
- Electric fans
- Flourescent desktop lamps
If YES that's the reason. Unplug them and keep them in a distance.
If NO then the length of monitor cable to the system, could be very long.
Reason & Tips:
- The new electrical devices could cause magnetic interferences to your monitor. Which will lead to colours in the corners, or jumping colour pixels or monitor shake.
- So whenever you attach a new electrical device, keep them in a reasonable distance.