I have a similar problem with Apple Tv. I have lots of films. Some are from Amazon, some are from DVDs and it doesn't seem to matter. The problem seems to be in the buffering of the movie from iTunes on my machine which is more than adequate (2.5 Ghz, 8GB 1333MHz running Lion). Here is my terrible, horrible not so good work around.
1) start the film on the Apple tv
2) press pause
3) when the progress bar shows that the film is completely downloaded (i.e., the progress bar is completely gray)
4) Press play
All things seem to work after that. FF, REW, PLAy and even the voice and video sync. This is not what I expect from Apple. It should be able to download and play at the same time just like it does with Netflix video streaming, but ... you know the rest.
I think the real issue is that they really don't want you to have any videos on any other devices but just download them from iCloud. Read the license agreement and you won't want to store any of your media in the Apple iCloud.
Hopefully, this helps.