problems withs mkv y avi hd

Hi
I am new to the forum. I'm from Spain and my English is poor. I'm using google translator.

My problem is this: when I play any mkv either 1080p or 720p every few minutes for the image to me and starts precaching and then continues.

I remember in previous versions it was all perfect but now ... I tried to change all the video options and nothing.

movies are hosted on a hard drive connected via USB to iMac and is connected via network atv2.

please help!!

thanks!