I have the same issue. Installed fios at the slingbox location (just for slingbox) have direct connect 75Mbps up and down (which is a crazy upload rate) and have 200Mbps download at the location in which I am slinging to my bitrate is 120kbps right now which is just embarassing. It used to be 5000+ kbps, no idea what happened.
I have a Slingbox in Massachusetts and I am watching it in Maine on a MacbookPro. I have Comcast cable/internet in MA with an upload speed of 5MGB and Time Warner internet with a download speed of 40MGB in ME. Also have high-end Netgear routers on both ends. I am getting a terribly poor bitrate speed and a resulting poor audio/video quality of only 200-400 kbps. At times, the bitrate speed will work its way up to as high as 5000 kbps, but it will only stay there for a few minutes and then it drops back down to the 200-400 level. I performed upload and download tests on both ends and it doesn't seem to be an issue with my cable providers on either end. Connection from cable box to sling box is via a very good component cable. Any thoughts on why I can't maintain stronger signal strength?