MacBook Pro with Retina display to Apple TV using Airplay is glitchy
Try this: open "Wireless Diagnostics" via Spotlight → Search/open Wireless Diagnostics → Window → Utilities and perform a wi-fi scan. Make sure that your router use a wireless channel with no/minimal other networks (you can change the channel via airport utility). If problem persists, try using the other wireless frequency (2.4ghz vs. 5ghz) via airport utility.

Next try streaming with Beamer app - much better video quality than airplay mirroring. Its a lovely app, and free to try. Solved all my airplay problems (have almost same setup; macbook retina + apple tv + airport extreme ac).

If problem persist, I guess I would try extending with an Airport Express (you can check the signal quality in wireless diagnostics).