I used to play Slacker a few times - to play a specific song - but that feature has been broken for a while (at least in my car), so I haven't been using Slacker - and I'm still getting all of the USB/media playback problems - requiring frequent reboots.
Last night, was playing a USB song - and the system stopped continuing to play the last note. I was able to recover by skipping back to the beginning of the song - and it played properly all of the way through. I've also had the system automatically skip forward to a new song or back to the beginning of the current song - while playing a USB song.
It's very likely the problem is not actually in the media playback software - but elsewhere in the system. Since the processor running the audio system is also doing other things, it could be there's a bug in one of the other "subsystems" that is causing interference with the audio software, which could make this type of problem more difficult to find - and could also be the reason why I've seen periodic problems with the navigation software - where it stops updating the screen or the navigation software begins displaying a different route in Google maps than what is being displayed in the navigation "dashboard".
When "sleep mode" was first implemented, it caused problems with the audio software - and disabling sleep mode fixed the problems. So far, I haven't been able to find any similar simple workaround to get the software back to being reliable.
Really wish Tesla would listen to their owners - and expand their beta to more of their customers. Among Tesla's "early adopter" customers for the Model S, there are a number of us with considerable background in developing and testing software - and could help them considerably during the testing phase in finding and diagnosing problems - when they have developers actively working on the software. I've tried to do what I can to help - by submitting many "bug reports" via e-mail or through the on board system, but by the time the software gets into production mode, most of their software team has likely moved on to the next major release - so fixing bugs appears to take longer (or they never get fixed in the current release, and we have to wait until the fold the fix into the next major release, like v7).