Uneven light-dark text - problem with monitor or video card?

Hi all,

I have the following setup:
monitor: AOC 2019Vwal 1680x1050 (new from Costco)
video card: eVGA e-GeForce FX5200 AGP Video Card - 128MB Light Version 128-A8-N303-L2
old Dell system with AGP 4x and PCI slots (guessing not PCI Express)

I'm running in the native monitor resolution 1680x1050.

Here's the problem: the text is unevenly light-dark-light-dark etc. It's very annoying and distracting.

It looks like different lines of text are light or dark in the same places left to right, as if there are dark and light vertical patches.

The problem is not visible at lower resolutions.

Is there anything I can do short of changing the hardware?

Is the problem primarily with the monitor or the card? I don't play games, just need very basic setup but want high resolution. What's the most cost-effective way to solve this?

Please help, thanks!