Vertical lines, background apps often show through on iMac screen (late 2012)

Hi there, an issue I've been having for over a year now, mostly on, sometimes off, is the appearance of subtle but disatracting lines and background app windows showing through foreground apps on my screen. This is especially prominent while watching videos or viewing darker-scale images full-screen.

I would assume it was just my monitor starting to go (it is almost seven years old) but the issue sometimes resides, but not reliably, after a restart or just at random.

What's interesting is, when I do a screengrab (Cmd+shift+4 style) the issue is not on the image. That means the screenshot is not truly grabbing what is reflected on the monitor, as the user sees it. I don't want to get to deep into that, but basically it's very hard to show the issue to anyone not in the room. The best I can do is a cameraphone snap, which always look awful.

In the one attached you can see the shadow of a Firefox address bar, showing through a video (Elmedia player here, but it doesn't matter) that I had full-screened. You can also see the vertical lines here, though it's somewhat obscured by the poor quality of a monitor pic.

In the past people have referred me to the "transparency" settings for mac OS, but this is not the issue. nor is it things left open that get burned into the screen. Whatever I have in the background presently, is what shows through -- plus I use a screensaver and have my monitor set to shut off after five minutes of non use. Any ideas?

Deets if needed:

iMac (27-inch, Late 2012)

3.2 GHz Intel Core i5

16 GB 1600 MHz DDR3

NVIDIA GeForce GTX 680MX 2 GB

MacOS Mojave: 10.14.6 (18G103)