Signal loss, I guess. The "server" is at the back of the house and gets a bit of loss. I haven't tried video clips and the like but DVD-quality Divx files stutter a lot.
I think the speed of the computer matters. Originally I didn't think so, as I thought "obviously the limited wireless bandwidth is the bottleneck here", but then I replaced my 'media server' (an old 4 year old laptop) for a new much faster laptop (with Windows 7), and things play great now, no stuttering at all, and they're all using the same wireless router. PS3, laptop and router are all in my bedroom though so that might be why I have no problems.