I can see two ways of trying to address this:-
1 - Create a domain network object for youtube.com (or which ever domain it is). Create a duplicate of your existing internet browsing rule and select this object as the destination endpoint. Apply an application defense, or application defense group (depending on whether you are using v8 or not) which does not enable AV scanning. Place this new rule immediately before your existing internet browsing rule. Normal web access will continue to use the rule with AV scanning enabled, but when you access YouTube, the new rule will be used which does not use AV.
2 - When you look at the MIME/Virus/Spyware settings in the application defense you will see, by default, it scans everything and depending on the size of the video and the available capacity of the Firewall this will result in the symptoms you are seeing. The file must be downloaded by the Firewall and scanned before it is then handed over to the client PC.
By editing these settings and creating a scanning rule which "Allows" (in other words, do not scan) the "video" MIME type, and place this scanning rule before the DEFAULT entry.
The Wiki page for Flash video indicates that there are number of different MIME types and file extensions which fall under the name "Flash", but if you wish you could create individual scanning rules to allow each MIME or extension type.
Hope that helps.