Cannot use nVidia drivers for FX5200

Having a Gainward FX5200 card in an XP PC with a A7V266-E motherboard.
Running VIA's latest 4in1 drivers. Problem is I cannot use nVidia
display drivers. Installing them, I get frequent long screen blackouts,
erratic mouse behavior and extreme sluggishness. I have even made a
clean XP install in the hope that it would help. It did not. The only
thing I can do to use the PC is uninstalling the nVidia drivers and
running with Windows' VGA drivers.

Any idea what might be wrong? Anyone seen behavior like this?

Sven