Poor video quality when mirroring on AppleTV. Don't know what to try anymore!

Hello everyone.

I keep encountering problems with my Apple TV.

I have a 27 inch iMac (late 2011) fully updated with Mountain Lion, iTunes updated as well and an AppleTV 3rd gen (updated also).

When i try to mirror my desktop on the AppleTV from the mac, the video qaulity is very bad. I can see pixels when mirroring videos (out of Quicktime, VLC for istance) all over.

I already had the AppleTV changes from Apple Support, but as soon as I tried it back home, I had the exact same problem.

I tried several things, like changing resolution on the iMac settings.

Could someone please help me? Does anyone have any idea on how to solve this problem?

Thanks in advance