Have experienced something similar to this with a client after they switched to using Macs. The symptoms looked remarkably like a network connection error but it wasn't.
In my case it turned out that when watching videos on the Mac (particularly flv), the Mac client would generated HTTP Header requests for portions (blocks) of the video file from the web server that is serving the video. If the web server ignored those HTTP Headers and just sent the whole video file down the wire then the Mac client would often close the request.
Our fix was to install Apache mod_xsendfile on our web server.
Unlike in Microsoft land where the client is happy to start receiving the whole video file, the Mac approach has more finesse because it allows users to scroll forward/backwards (file seek) easier/quicker and is more sympathetic to mobile connections and data costs.
Unfortunately not all web servers on the internet support Apple's HTTP Header requests for video files.