Today I did much testing with different devices chromecast or not, phone, tablet, pc, on wifi and on cellular, setting up a new trial account, and had the same disconnecting in all cases. I listened to songza (owned by google) and had the same timeout/disconnecting issue and last I listened to rdio and have streamed for 5 hours straight with no interruptions.
I called google back and this time they told me that it's recently acknowledged known issue, and they are working to resolve it on their end. I wish i would've gotten that info the first time I called, but I'm ok with it and just happy to get 24 hour US tech support. I understand that they have troubleshoot every issue so they don't end up with someone waiting for a problem to get fixed by google if it's a problem with the hardware, network, operator error, etc. So if you're having the issue with google play music all access dropping connection randomly, or automatically pausing itself after 5 seconds, it may be the same problem.
I'd be curious to hear if anyone is able to stream music without any intervention (skipping, browsing, etc.) for longer than 1.5 hours. I would think it would be all over the forums if it was a global problem, but the tech support agent I talked to said it's happening to him too.