Pablo, when using the player, there are two versions of each feed: 1 Flash and 2 Flash. At least one of those is working at all times. When you click on a channel, one of those feeds is buffering. Check the status bar to see which version of the Flash feed you are going to get.
The status bar is between the video area and the channel selection area.
It will read either:
Status (1 FLASH): [Idle, buffering or playing]
or
Status (2 FLASH): [Idle, buffering or playing]