Faint horizontal lines across screen after installing new video card

"First thing to do is establish whether these lines could be drawn by software/video logic, or if they can only be a result of interference or other low-level analog/electronic stuff. If it looks like something you could possibly, but realistically and COMPLETELY ACCURATELY AND TO SCALE, draw in photoshop given enough skill, then it's more likely to be a driver issue than a hardware issue. Try upgrading the drivers and card firmware.

It's not clear if you're plugging the second monitor into the same video card connector. If the other monitor shows the same problem in the same video mode, then get the card replaced. If you can't test with the same connection and same video mode, borrow a monitor and test it, or have the retailer show you it working on a different monitor. If you can't prove that it works on another monitor, get the card replaced, definitely.

See if this happens in a different video mode (lower resolution). Try replacing the video cable with a high-quality one that has shielding. It should be a thick, not too flexible cable). It should also have the bulky cylindrical ferrite cores on each end to reduce interference. Don't worry about crap like gold connectors, though they won't do (much) harm.

Clearly if only one monitor is bad, and you're using the same cables to connect each (which you're not if a monitor has a permanently attached cable), then it's more likely a problem with the monitor. You can try plugging the monitor directly into the mains if you're currently powering it through the CPU. Since different monitors have different power requirements, it's worth doing this even if only one monitor shows the problem.

If you don't want to replace the card, you could probably work around it by changing the sleep options on the computer and/or monitor. Suspend to disk (hibernate) might be a viable solution, as it should fully reset the card."