Agreed, 12.3R12 is the way to go currently. I had upgraded a few of our sites to 15.1R5.5 (latest) and all day every day while that code was present,there were chassisd log messages. The VC would lose connection (not sure why) and Space would shoot us a DOWN notification. Few minutes later, chassis would once again recover. Rinse and repeat.
Downgraded one site this past friday back to 12.3R12.4 and nothing in logs, silent, works flawlessly. 15.1 is a POS right now. That's my .02.