I have a Premium account and use it often with my iPhone and car radio adapter. My problem started a couple months ago, but has been happening more frequently over the past couple weeks (or maybe I've just noticed it since traveling for the holidays).
The problem is that the app will randomly pause/unpause, and often Siri will pop up without me touching my phone. Sometimes this several times a song for several songs in a row, and sometimes it will just be once every few songs. Shutting off my phone and turning it back on sometimes helps, but sometimes does not. The funny thing is, this only happens when hooked up to my adapter, never when I'm using Spotify and it's not hooked up to anything.
I have an iPhone 5, version 6.0.1 on Verizon, and app version 0.5.9.10. Although this happens more frequently now, I also had this problem sporadically on my old iPhone 3GS on AT&T, so I don't think it has anything to do with my actual phone, provider or app version.
Please let me know if you've ever had this problem! It's getting to be annoying and I love Spotify otherwise, so I'd hate to have to get rid of my Premium account because of this!