Happy birthday TS,
from wat i observe maybe is your GC problem.
i have that kind of experience.
What you can do is...

1st, take out your GC and use a blower to clean the dust..maybe it is the dust problem.

2nd, try take our your GC and use the integrated on board VGA and see whether the problem persist.

3rd. check your VGA cable, maybe its too old and unable to trasmit data.

4th. Maybe is your monitor near the end of life.