What GFX Card you using?
edit: I'm finishing work in a minute and heading to the airport a few hours after so I won't have time to respond so I'll update this.
If your trying to run 720/1080 with the onboard gfx card that's your issue, it just won't do it. You need something like this
£20 Zotec nVidia 210 or similar (or better, long as its low profile).
If you are using something like that already and its still jumpy etc then make sure that DXVA(?) is enabled in XBMC > Video settings.