How much memory do you have and also what memory does your graphics card have? Minimum RAM will have a tendancy to make video playback a little jumpy especially when other programs are open and using memory.
I find I have no problem with 512mb Ram minimum and 250mb Ram on the graphics card. With that I can usually have 10 - 15 IE windows open, a Private network running and 2 graphics programs plus windows explorer and the email client.
If your memory is fine then I would check your swapfile settings.
I have got the same problem, but on a mesh matrix desktop pc. It has got 1gb of ram. the playback is even jumpy in websites if I try a streaming video or it has a running graphics update like a news site. I have been advised to take the pc back to factory settings by mesh. I have had this problem since I got the PC.