iTunes 11 Video Playback Stutters

Hello,

I have just bought a new Mac Mini to use in the living room, one aim is to replace the Apple TV I have been using to stream media to my TV. The details:

Mac Mini connected to network via HomePlugs/Powerline (85). iTunes library on a NAS that is plugged into my router. Previously I had an ATV2 in place of the Mac Mini, the copy of iTunes that it used was on my Mac Pro with the library on the same NAS.

While playback wasn't always perfect with the ATV2 setup and sometimes I would have to pause a film to let it buffer enough this setup lasted me for ages without real problems. Playback via iTunes on the Mac Mini however is a mess with constant stuttering/audio out of sync etc. I am going to upgrade the homeplugs as a matter of course anyway but find it hard to believe they would suddenly be causing the problem, does anyone have any ideas