I'm already on the preview program and it's still happening. The only real fix is when it starts disconnecting and acting weird, you need to close spotify then go in app settings, kill spotify and then clear cache of spotify. Then reload spotify and voilà. Annoying but it takes 20 secs to fix. When it starts to bug again, do the samething.
Joined the programme as you suggested, I am now on the same firmware version and also using the Chromecast 2. Still the same issues. I can't believe Spotify does not address this issue which people seem to have since months. This is a bad joke.
Same problem here, and they seem to be doing nothing to fix the issue. I've gone around and around with their tech support on chat and the @spotifycares (hilarious handle btw) on Twitter to no avail. They keep suggesting the dumbest most obvious things to try to resolve it (is your wifi connection ok? Why don't you try reinstalling the app?) instead of working on an actual solution.
So somone on Twitter said to disconnect Facebook from Spotify using the desktop spotify program. I did that and so far my dropouts have stopped. Also the Android app update on Oct 19 has seemed to improve the speed and reliability of chromecasting and switching.