I do have a similer issue.
I see 2 possibilities from my experience.
1. ISP was limiting the uploadspeed for streaming service such as Sling (fixed it by changing ISP)
2. Sling software just have bunch of problems....too many bugs. It happens to me every once in a while (especially when I try connecting with new device, such as rovue and sony google TV especially messes sling pro HD A LOT!!!
I generally fix it by connecting with multiple computers and from difference language sling websites.
For somereason when I do that, it fixes the issue, and I can go back to view online with my regular Windows 7, chrome based player.
Do you get both picture and sound? Just a lot bitrate issue?
By the way, the link from sling admin wont help my issue other than ISP limiting the bandwidth for sling.
I think many of the problem people are experiencing are all sling's problems, such as the notorious capacitor blowing and software bug issues.
Once you get it working, it's a great product so good luck!!
I am actually hoping for the next version to be able to raise the upload speed since no matter how fast your upload bandwidth is, I think sling cannot go above much more than 3000kbps.
Also hoping the encoding technology gets better since this codec is so weak for fast movements.