Vista - horizontal interference at max resolution

Hi All: I recently upgraded from XP Pro sp2 to Vista.

I have two cards installed: a Gigabit AGP 8x card and an ATI Radeon 7000
PCI, neither of which has Vista drivers or is on the compatibility list.
Under display settings, Vista selected the Radeon 9250 driver for the Radeon
card, and "Standard VGA Graphics Adapter" for the Gigabyte AGP card.

On XP, I was running both monitors (an NEC 17" LCD connected to the Gigabyte
card and a Rosewill 19" LCD on the Radeon) at 1280 x 1024 resolution without
a problem. When I set the monitors to use the same resolution in Vista, the
Rosewill started to show horizontal flickering bands of varying intensity,
regardless of which card it was connected to. Dropping the resolution to
1024x768 eliminated the interference.

Question is, is this a display card, driver, monitor or refresh
rate/resolution issue? Seems like the only difference is the driver being
used, and indirectly the frequency/refresh rates supported by it?

Is there any way around this issue, without appropriate drivers? Do I need
to / should I upgrade the video cards to Vista-certified models? I'm hoping
to avoid that expense, but I prefer running the monitors at a higher
resolution.

TIA!