I've been seeing this problem for a little while now, and I believe I have narrowed down the problem to one particular settings.
I have a 1GB network in my home via Linksys E3000 router.
When I try and play my slingbox over my network in BEST mode, I get data rates of 8000 kbps, everything is great.
However as soon as I switch to BEST HD mode, the connection rate begins to drop, and at around 100 kbps, the video goes black and eventually drops to around 30 to 40 kbps where I lose audio and video.
If I change video mode back to BEST (not HD) the rate jumps back up, and video/Audio once again resumes.
This behavior occurs when viewing via Browser (IE9 and Chrome) and when using the Slingbox Windows Standalone player.
I've also tested this on various computers within my household with different browsers (e.g. IE8 on my Laptop) and the same thing occurs on every one.
I've owned my Slingbox Pro for a couple of years now, and this issue has only started happening within the last 4 months.
Feedback greatly appreciated.