From my observation when the dropouts occur my Macbook Pro is not in 'sleep' as such (i.e. the front LED is not blinking), but it has gone into a suspense mode where the display has gone blank and the user password is required to regain access.
Despite the fact that that the machine does not appear to be in sleep mode I have found that going to system preferences and setting the 'computer sleep' option to 'never' is an effective temporary solution to this problem (see below).
When I was using the previous version of iTunes it would never allow the machine to go to sleep when the Apple TV was accessing the shared library. It appears as if the new iTunes has introduced a bug whereby the system thinks it's going to sleep and cuts off the shared library access as a result.