Streaming movies doesn't work

When streaming video podcast and movies at the Apple TV from my iMac it works perfect - even with high quality video podcasts but when streaming a movie that isn’t in itunes it doesn’t work. It is to slow and the movie isn’t worth looking at.

Any advice? It was my main reason to buy AtvFlash so i didn’t have to convert all the movie formats and add them to iTunes

Thank you

Do you know the specs for the file you are attempting to play?