Deskpro EN SFF graphic problem

I bought a deskpro P3 733mhz with 16mb onboard agp graphics 128mb ram.

I am running xp pro on it but the graphics stutter. Thinking that the
onboard vga was not up to the job I installed a TNT2 M64 16mb PCI card but
get the same thing. If you run dxdiag it shows the bouncing square
stuttering every few seconds.

Anyone know of a fix for this? I checked the hard drive and its running in
dma mode 5 so no problem there!

The people I bought it from have recreated this fault on their remaining
machines so its not my own machine that is faulty. They cant seem to find an
answer either but my own guess is that the chipset drivers need something?