I was having this problem too. My setup is HP Notebook running Windows Vista Home Premium streaming music, etc. in iTunes using Homesharing via wireless connection to Netgear WNDR3400v2 router and Ethernet connection to Apple TV 2.
Finally in frustration what I did was uninstall Apple software from my computer (iTunes, Bonjour, Apple Software Update, Apple Mobile Device Support and Apple Application Support). I left Apple Quicktime installed. I then downloaded and reinstalled the latest version of iTunes (10.7.0.21).
I disconnected power, HDMI and Ethernet cable from the Apple TV 2 for a minute then reconnected all.
I have no idea which if any of these steps resolved the problem but it has been eliminated so far (about a week now).