I had something similar happen. I also have the grandfathered unlimited plan and use my slingbox all the time. For some reason slingbox would buffer even when other video intensive apps would stream fine. I called AT&T and asked if they were slowing video only down on me. After dicking around they finally said yes. After that day my downloads have been mostly buffer free. That and the fact they just announced they will throttle after 22 GB instead of 5 GB has helped. Also try resetting the slingbox if a talk with ATT doesn't help. I had to do that once to get the buffering to stop as well.