That introduces a panapoly of potentials, unfortunately. You might have a duplicate IP, or the amount of data going in to the line-in my be flooding the ability of the system to process it. Does it occur when you're not using the line-in? If so, it's likely a duplicate IP address. If not, then I'd suggest changing the line-in to "Compressed".
There's actually a possibility of wireless interference on the PLAY:1 as well, and there's a FAQ for that. Basically, look for things that might be generating interference, try changing your wifi channel, either on the router or if you're using SonosNet.
Bruce