I had the very same behaviour with any 3.17+ kernel: sometimes it doesn't
crashes for days, others it crashes multiple times per minute. It doesn't
matter what you do, it just crashes (even starting your desktop environment is
enough sometimes).
I will probably buy a Nvidia GTX 970 while waiting for the new unified driver,
then I will try the open source path once again: hopefully having the
proprietary driver using the very same kernel code will force AMD to take
stability into higher consideration.