Abit VP6 with VIA based SATA card, works depending on gfx card??

Hello,

OK I know old hardware is temperamental but I'm curious and not sure where to go with this.

Problem: I have an old work horse dual P3, abit vp6, matrox parhelia gfx, and a few other bits n pieces. Some time ago I put in some cheap via based pci sata card so I could use a single sata disk (card is RAID-able but I have no interest in such things). This all works fine - i.e. just following POST the VIA RAID bios screen pops up, presumably makes the MB bios aware of my "scsi" disk and the machine boots.

I wanted to change my gfx card as the parhelia doesn't seem to want to run two screen at different resolutions (I just bought an HD screen). So I plug in my (AGP lol) Radeon HD 3650 and fire up. POST ok but no VIA RAID bios screen and no boot since the MB bios was presumably unaware of my hard disk I pulled every other component out on the chance that I'd created a conflict, tried a multitude if bios IRQ settings etc to no avail.

I tried a different gfx card, some old Geforce I had lying around. Same problem.

Thinking that maybe the old dog had given up I stuck the matrox back in and it all worked fine again - no need to change any bios settings, boot order, IRQ selection etc etc.

So, somewhat of a typical ancient hardware problem I guess BUT anyone got the slightest idea what those other gfx cards do that prevents the VIA bios kicking in when the matrox doesn't?? (and while it of course is not the end of the world I'd really like to solve it )

Thanks