I had the same problem for months until I found a hint in a forum (Author Big Worm) which gave me an idea.
The problem is in the router/networking area and has to do with package size of data, etc. which I don't understand anyway.
But....it was suggested to disable a router function IPGM, which I could not find in my Asus N55U.
So I looked on the Mac under system preferences....network....ethernet....advanced.....TCP/IP....Configure IPv6, which I set from automatic to link-local only.
BAM...I had immediately the connection to the itunes libraries.
ATV 2 now works, also Airplay. Only glitch left is: when I use my airplay loudspeakers in connection with an IPad and then try to use the loudspeakers with ATV, then the connection to the itunes libraries collapses.
But since I know that I can work around it.
Maybe that gives you a hint..
Best regards from the snowy allgaeu alps.
Peter Schmucker, Kisslegg