I vaguely recall encountering something like this. The problem goes away if you plug your monitor directly into your primary card instead of the Voodoo2, right? If so, you could always get some kind of VGA switch and manually switch to the Voodoo2 when you need to rather than relying on the passthrough.

If I had to take a wild guess, when your motherboard initializes the card, it starts outputting some kind of signal that interferes with the signal from your other card.

Maybe there's some mini DOS glide diagnostic app you could put in your autoexec.bat that would effectively switch to the Voodoo2 and back again?

Yeah that's what I am looking for , some app to do a switch test or a way to load the control panel dll and make it reset the switch (but that would be asking too much 🤣). And the image straight from the VGA is crystal clear.
I don't really want to use an external switch , I don't like them or their cable mess or their manual nature... 😀 If it comes to that I can use the monitor's 2nd input but quite frankly I'll ditch the voodoo2 before I resort to that...

Something from the SDKs or diagnostic kits at http://www.falconfly.de/reference.htm , perhaps?

(I have no idea what in particular might do the trick, but no one else is jumping in here.)

ah thanks for the link Jorpho , I'll check the files inside and I hope there is something I can use 😀

I have looked in the diag/utils stuff on the sdk packs but sadly none of these did the "reset" I am looking for... apparently it's not enough to start a glide app to get rid of the moving lines, the only thing that gets rid of them is opening the display properties... strange....

Strange indeed. There might be some way to launch the relevant control panel using Rundll32 (see http://vlaurie.com/computers2/Articles/rundll32.htm and http://www.robvanderwoude.com/rundll.php , for example), but it might take some digging (maybe with the old Filemon that sysinternals.com used to offer) to find out the exact command.

Otherwise, maybe just moving your Voodoo2 to a different PCI slot might do the trick? It's worth a shot if the alternative is doing without entirely.

You know, the first thing I tried was rundll32 🤣 I even looked in the control panel dll to get clues of a possible entrypoint that might do the trick 🤣
The pci slot is a nice idea but unfortunately the HOT591 mobo doesn't offer much choice: it's the last one or none since a lengthy card just won't fit anywhere else (cpu placement is to blame , it's aftet rhe 3 pci slots....)