Why does my apple tv freeze when mirroring with my macbook pro?

I recently bought an Apple TV unit to go with my Macbook Pro (Summer 2012, non-retina) that has been updated to Mountain Lion. I was excited about using the mirroring feature so that I could stream movies from my computer and watch them on my TV (especially movies for a class that are posted online through my university's library services). However, when I try to mirror my Macbook with the Apple TV, the unit responds extremely slowly--it will freeze on a frame and then jump to another freeze every minute or so. I was able to fix this once by restarting the unit, but for whatever reason, that is no longer working for me. I mirror fine with my iPhone--no problems at all there--and that's what leads me to believe this is not a network issue. I'm quite frustrated and would love it if someone out there could offer me some advice.