Thanks for the response. I was connected to a TV PBS website to watch a movie.
It started fine but when I went to "closed captions" and some other options it froze and I was unable to continue playing it, or resetting it to the beginning, by using the slider, etc.
I started all over OK but again had the same problems.
I was not using my iMac for this. I was using my MacBook Pro, Mid 2009 variety, with Safari 6.1.
The "allow plug ins" was actvated before watching.
I was having problems just this week -- the PBS website streaming worked better than others (Netflix was simply not downloading movies at all, and even my own travel "movies" would stop loading early in the streaming process, and take forever to complete)., however it (PBS) would stop to show a Ralph Lauren sponsorship message, and then start all over again! Having searched these forums, I noticed several others were having similar problems, but I wasn't seeing any answers.
I went to the Apple TV settings and started at the top, just checking about everything to see if there appeared to be anything I needed to readjust (as there was a fairly recent Apple TV software update). Nothing made a lot of sense to me, but at the bottom of the list there was a "Restart" option, so I clicked it. It really didn't take long, and Lo and behold: I have since been able to download Netflix stuff, play my own videos, and also view PBS programs without a hitch. SO -- I suggest you try the "restart" from your Apple TV Settings menu!