I know there was a problem with AV sync a few months ago with the Sling plugin; at some point things stopped being problematic. However, it appears as if the problem has returned...and is doing it to an even more severe and predictable pattern.
The computer I'm doing this on has a Core i3-380M processor, 4gb ram, Windows 7 64-bit, and is connected to a Sharp Aquos TV via HDMI configured as the primary display at a resolution of 1920x1080 with RGB colorspace. I have tried using both Chrome and Internet Explorer with the results being exactly the same. I have also tried this on a ProHD located remotely and a 350 located locally. My internet connection is a 75/35 FiOS connection; and the computer is connected to my second wireless network; which is a 300mbps 40mhz wireless-n station with virtually no traffic. The network is indeed running at close to it's 150mbps per-stream; and my internet connection is solid at an approximate 82/37mbps.
On the ProHD, after a few hours of streaming in full-screen the AV will become out of sync by 3 or 4 seconds. If you switch out of full-screen mode, the video renders at a very fast rate and resyncs with the audio. However, putting it in to full-screen mode results in an instant lost of sync. This behavior was the same between both IE and Chrome.
The 350 is a slightly different story. Throwing it in full-screen immedately results in a loss of sync; however exiting full-screen mode does not allow the video stream to catch up to the audio as it does on the ProHD.
It's sad to see this start happening again, espeically after the problem seems to have been fixed in the past; and considering the browser plugin produces a much better video output with the ProHD than the dedicated SlingPlayer app.
Make sure all the drivers on your computer are up to date. If you find they are all up to date, and issue continues then please look at this helpfull article linked below it should be able to assist you with your issue.
Sling Media Moderation Team