Help with Matrox G400

Please help me!

I played UT on my computer AND on another computer with the same model of video card (Matrox G400 32Mb SH)and, on both systems, I noticed heavy polygons problems. The polygons disappear, have many irregular forms and so on...

Did you notice the same problem? What can I do?

Sorry, I forgot to say that I have the latest drivers installed, the version 402 (but the problem was also in the demo and the unpatched version), the DirectX 7.0 (but I tried also the 6.1).
I have a PII 333 Mhz with 128 Mb.