Horizontal lines moving vertically down my screen

This will be kind of hard to explain, so bare with me. This is very hard to notice in well-lit areas, but it's easy to spot with a dark background or when I have my character menu or passive tree open. There will be very transparent horizontal lines stretching across the entirety of my screen, moving down and usually fading after a few inches. Like I said, they are kind of hard to see but now that I noticed it, I can't take my eyes off it.

It doesn't just happen in PoE, but I really don't know where else to ask and the help you can expect from these forums is great. PoE is the only game I play, but I decided to try in a couple other games and noticed it even more in those games. The best example I can give of what this looks like is when you would see a CRT monitor in a video recorded with an old camcorder, although the effects are nowhere near as pronounced.

I am using an nvidia GTS 250 and my drivers are up to date. My first thought was that it was monitor-related, but the only time it happens is when in a game. Thanks for any replies!

have you turned on V-sync?
if not this should sort that probem

RAM going bad can cause that issue.