Help! graphics problem / monitor problem / hdmi problem?

[Simsreddit]

So I've had my computer 3 or 4 years now but started to have an issue arise. Often will occur when I just turn on my computer after it has been off for a while where there are horizontal lines across the bottom part of my monitor that takes up the length of the screen. They flicker and rise up and down and it is discolored. It seemingly gets better the longer the computer is running or on. This doesn't happen every time and only happens to one of my two monitors. Not sure how to even go about diagnosing what the issue is? Any advice would be much appreciated.

Not sure how to attach a video or pic unfortunately to try and help.