Greg - I am having similar issues - I have tried a dozen HD formats - DivX, ASF, H.264, XVid, WMV, etc. No better. All using Media Center and Win 7 RC. I too have tired the wired vs wireless test - no change. Running the network tuning on Media Center indicated a bandwidth issue (100MB wired and 802.11n wireless)....
Rick
Update - used the gSpot codex to check for packed files - not the case. Still having jumpy / sluggish playback, but on Blu-Ray files...regular video plays perfectly. Not sure if I have a network bandwidth issue or Media Center can't pump enough bits. Processor is barely working.