I had experienced this issue for approximately 1-2 months and below is my steps to clearing the issue. Stream was under 100 kbps and the video was laggy to the point you could not watch anything. Stream end has around 4 mb upload and receive end has over 10 mb download.
Browser: Firefox 3.6.12
OS: Windows 7 64-bit
Slingbox firmware version: 2.1.100
Slingbox hardware version: 1.34.3
WebSlingPlayer plugin version: 184.108.40.206
1. Go to "Tools" and click on "Addons".
2. First thing to do is uninstall the SlingPlayer plugin.
3. Go to the "Extensions" section in the top.
4. Scroll down to "WebSlingPlayer" and click it. You will now have an option to uninstall. Do so now.
5. Restart Firefox.
6. Go back to the "Addons" popout.
7. Click on "Extensions" again.
8. You will most likely have multiple versions of "Java Console" extensions installed.
9. Disable all but the most current one (as of this message it's 6.0.21). You can take this time to disable other extensions that you know you don't use any longer.
10. Restart Firefox.
11. Go to your Slingbox and run the web application and go through the proceedures to reinstall the plugin.
That was my fix. It took it about 5 minutes to get fully cleaned up but I went from under 100 kbps to my previous normal of over 500 kbps. I would suspect a similar solution could be completed on IE. Hope that helps anyone that has the same issue.