Monitor trouble

Earlier tonight, my computer monitor starting displaying these translucent gray bars of varying width across the screen. Opening more windows creates more bars.

The diagnostics swear things are working properly. I have disconnected and reconnected the cable leading from the monitor to the computer tower, with no discernable change.

Anyone have ideas as to what might be going on? I don’t have a spare monitor to swap to see if it a monitor problem or a video card problem.

I should add that lines of text on a webpage seem particularly conducive to creating these gray bars.

Horizontal bars or vertical bars? And do they line up with the edges of the windows?