It not Mountain Lion. I am running Lion.
I suspect it is either the latest vesion of iTunes 10.7 or the latest Airport Express 802.11n (2nd Gen).
I have a Cat 5 wired house, and only use 3 AirPort Express 802.11n (2nd Gen) for streaming music using the analogue outputs. All streamed content is cutting out after between 35 to 55 minutes from a contunual feed.
So far the only way to resolve the issue has been to reset the AirPort Express and then streaming is fine for another 35 to 55 minutes whereupon it is necessarily for reset the AirPort Express again.
Hopefully a fix is coming, as it appears to be a common issue looking at all the other threads on this forum...
This may be a solution for your Airport Express Cutouts if you are using APE as Network Extender.
Not sure if this is a permanent soultion, but so far it's working extremely well for me.
My Basic Set Up:
Airport Express (x2)
With the 2 Airport Express units I was expericencing AirPlay cut outs very frequently. Originally both units were set up as "Network Extenders". I live in a 2,500 sq/ft home and as mentioned, I also utilize a Time Capsule.
When I set up an APE next to my iHome in the bedroom, I thought "Awesome, I have a WiFi extender next to my bed... this must mean improved signal and better speeds on apple TV and ipad, iphone, etc." ---- WRONG!
I now believe having both APE units set up as wireless network extenders caused confusion and interference amongst all our devices. Truth is, I think Time Capsle is powerful enough to transmit a clean strong signal throughout my entire home. APE as an extender might be good if your using APE as a relay point to get a signal out in the backyard, for example.
The solution: I reconfugured both APE units to just "join existing network" and as a result I think I eliminated confusion and interference amongst the various signals.... Keeping it simple the way Apple intended.
If you think about it... the signal from APE is going to be weakend if it's the "middle man" between your Time Capsule and your laptop/phone/ipad/etc. So my theory is that since I had a APE next to my bed, which is about as far from Time Capsule Base Station as I could get, why would the signal be stronger to that unit than it is to my laptop 2 feet away? It's an extender, not a booster (unless APE hardwired which mine is not). I think my phone/ipad/apple TV was recieving the watered down signal from APE and/or conflicted about what device to get it's internet feed from.
Hope that helps...