I ubergnome

I had a quick look at the specs. on your machine and wondered if it had 1GB RAM or have you upgraded it to higher. Also that spec. showed the rig used a Intel GMA X3100 Graphics card. As you may or may not know this is an integrated graphics card that doesn't feature its own memory but can address up to 384MB of system RAM for video memory. Reviews on this card show a low base score in Windows Experience.
In conclusion it might be that your problems may be partly down to having the minimum 1GB RAM recommended for running vista (other than home basic) coupled by the extra "drain" of your video card borrowing up to 348MB of system RAM. This could cause the stops & jerks you mention. Why it should be only in WMP11, I'm not too sure but maybe this uses more RAM than other applications.
Solution....Well if you are sure it is not a driver problem, I would be looking at possibly upgrading the memory RAM to 2GB.
P.S. I ran vista on a Dell Inspiron with integrated graphics and found that 1GB of Ram not sufficient to get a decent video performance!!!
Anyone else had the same problem???????????