I just bought a Pro HD two days ago and after a lot of fiddling, I think I have figured out some things about jittering/studdering.
I found that if I hit the pause button on my DVR, the jittering would stop for a bit, and then start again. So I figured that maybe my computer, even though it is above the spec line Slingbox publishes, was the issue.
I went to the options menu and went to manual settings. I had already lowered the resolution to my native resolution, so I pulled the stream rate down to 3,000. I set the audio bit rate at 64.
This has made all the difference, without a noticeable (to me) change in resolution. I have watched football for an extended period with no jittering. Great action and resolution. It's even working on a sony netbook I have with a 2.0GHZ Atom.
Took some experimenting to get to the right stream rate for my laptop.