The first thing to check would be your internet download speed, you can do this at www.speedtest.net. If you do this using a computer connected by ethernet, you should get a fairly good indication of your incoming speed from the ISP. If you are using a device connected by wifi you will need to take account of the portion of your internal network being used and the effect it might have on the result you are seeing. Another option is to access your modem (see your manual to determine how to do this) through your web browser or specialist software supplied by your ISP, the software on the modem itself will often tell you your connection speeds somewhere.
1080p HD movies require a recommended speed of 8 Mbps, 720p HD movies require a recommended speed of 6 Mbps, while SD movies require a recommended speed of 2.5 Mbps.
Intermittent problems are often a result of interference. Interference can be caused by other networks in the neighbourhood or from household electrical items.
You can download and install iStumbler (NetStumbler for windows users) to help you see which channels are used by neighbouring networks so that you can avoid them, but iStumbler will not see household items.
Refer to your router manual for instructions on changing your wifi channel or adjusting your multicast rate.
There are other types of problems that can affect networks, but this is by far the most common, hence worth mentioning first. Networks that have inherent issues can be seen to work differently with different versions of the same software. You might also try moving the Apple TV away from other electrical equipment.
The following article(s) may help you.
Troubleshooting Wi-Fi networks and connections
Recommended Wi-Fi settings
Sources of Interference
Wifi Diagnostic Software (for Mac users)
You may also find some help on this page, where I’ve collected some of the more unusual solutions to network issues.