I'm experiencing this same problem, I think. All of a sudden, my SlingPlayer clients are experiencing peaks and valleys with bandwidth. It will run high quality video at 3-4M for around 5-10 seconds, then the playback will freeze and the client says I now only have 200K bandwidth. Then the speed will build back up to 3-4M and the process repeats itself. This problem just started in the past week or so.
I have not tried manually setting the bandwidth to see if that fixxes the issue, but I will be doing that next.