Three possibilities to test:
(1) Verify that the network connection settings on your computers are optimized for the speed of your ISP. A simple and safe way to do this is to download the free program TCPOptimizer from http://www.speedguide.net/downloads.php and apply the Optimal settings for your speed of connection, by moving the sliding bar near the top of program screen to the advertised download speed of your ISP. Click the boxes to apply the optimal settings to all of your adapters, then apply the optimal settings and reboot your computer (I am assuming you are using a PC and not a Macintosh).
After rebooting, see if the situation improves.
(2) login to your router and check the settings for something that might be called "WMM" or "Multi-stream" or "Quality of Service" -- these options are used by the router to determine how much of the bandwidth to provide to media devices attached to the router, with the intent to provide all devices connected to the router a "fair shake" of the bandwidth. If you find any of these options on your router, try changing their settings from "enable" to "disable", or vice versa, depending on what the current settings are. Apply these settings, then reboot the router and see if you notice any difference in the video streaming or the ability of your browsers to access the internet
(3) when using SlingPlayer software on some of my older PCs, I found that I could prevent occasional stuttering of the video or audio by opening the "Settings-SlingPlayer Options-Encoding" menu and changing the Intenet Access drop-down menu from its default "Automatic" setting to a lower video resolution, such as 640x480. Since the video source for my Slingbox is providing Standard Definition video only, this lower setting does not seem to degrade the video image, but possibly reduces the amount of data that the Slingbox is having to push through my Internet connection.