This might not be related at all, but I have a slightly similar issue with a cheap little flat panel monitor, except my lines are vertical. They are different technologies, so it may not apply, but I figured I’d share just in case.

It seems like my monitor won’t properly adjust its timings for a clean image. If I go into the menu and tweak things manually I can fix it. So maybe poke through the OSD and see if fiddling with anything changes the lines. At least then you’ll have an idea if it’s the card or the display.

I also found out that this only happen when i run 1600x1200. When i lower the resolution anything is fine..

Have you tried changing the screen refresh rate for the 1600x1200 resolution?

Jackhead wrote on 2023-02-15, 18:27:

When i start a 3dfx glide game like Quake 1 or 2 (V5500 1600x1200x32) i had this lines on screen. When i switch to openGL they gone.

Quake I (GLQuake) and II never used Glide natively. It was only done thru the 3dfx MiniGL wrapper for early Voodoo cards.

found the problem, i had to switch in the 3dfx settings from Performance to AA+2.