Can you post a picture of the issue?
What resolution and refresh rate are you running when the problem occurs and what screen are you using?

PS: The RAMDAC is integrated into the chip. You can trace the signal way starting at the VGA port to the chip but noise can enter the signal anywhere along that way.

Vertical lines on the screen are actually quite normal for many old VGA cards when connected to a modern LCD. There's nothing you can do, apart from trying a different LCD.

I own the same (or at least a very similar) card. It also has hard-to-ignore vertical lines on my LCD monitor (a Dell UltraSharp 2001FP).

Or a CRT if you can snag one. The Cirrus Logic 542x cards are typically very solid in 2d. Especially the 5428 imo.

I'm using a FlexScan from Eizo. Here some photos. The vertical lines are more visible when the background is dark and/or unicolour.

So I guess I'll have to live with it or check for later ISA VGA cards.

normally Cirrus Logic Cards are make one of the best pictures on TFTs amongst the ISA Cards.

Install CirrusLogic Mode tools and set proper Frequencys first.