Noisy output, a common problem of old ISA VGA cards in general. If I had to guess it's due to the layout of the output DAC / clock circuitry and/or routing of power traces. Recapping almost never fixes the issue.
Interestingly, this seems less of an issue on CRT monitors, just on old VGA cards trying to output to a flatpanel.

Yes, this is also known for other ISA cards connected to LCDs, but Trident is famous for it. It has names like spaghetti lines, jail bars and so. It was also discussed several times here on the Vogons, for example:
ISA VGA output crap on LCD?

My SiS 5598 does this on my LCD but only at 640x480

You can try changing the RAMDAC (the HMC labeled chip) to something like a Music TR9C1710 or Sierra SC11487CN or equivalent 28-pin DIP DAC, but that's not guaranteed to fix this issue. Sometimes it helps a bit though.
Higher-quality RAMDACs* can also reduce the sparkling artifacts that often occur when the palette registers are written (Wolfenstein 3-D, Solar Winds title screen, many other games).

* Meaning those with flicker filter circuitry. Some very old but otherwise quite high quality RAMDACs (inmos, earlier Analog Devices parts, etc.) flicker because they lack such filtering

Apparently these don't show up on a real CRT. I usually don't use LCD screens with VGA cards. My ancient Dell 1280x1024 LCD really struggles to lock on to the analogue VGA signals from the "low end" ISA and PCI cards like the generic no-name OEM stuff that uses S3 and Trident chipsets, even though the output from those cards is pretty much perfect on an actual vintage SVGA CRT.

Your card has a socketed RAMDAC so it'd be easy to try a different RAMDAC and see if that improves things. Here's another recent VOGONS thread with some promising results:
Let's improve video output quality of VGA ISA/VLB cards (First success !)

Thanks everyone, I finally found a CRT monitor and tried both cards, absolutely no problem with them.
Weird thing though, neither card works on my WD2 286 motherboard. If I put them in, the motherboard refuses to boot. If I use a Cirrus logic card it works fine. I guess the motherboard isn't compatible with trident cards?