I've only seen that happen when the GPU itself is locked up.

The phoenix process cannot be killed, because it can no longer talk to the GPU.

Are you overclocking?  The GPU locking up is basically instability due to overclocking.  You may be *right* on the edge of what the card can handle, so when anything "strange" happens, it locks up.

Try backing the GPU down 5-10 MHz and see if it stabilizes.  Have you noticed it always happening to the same device?  e.g. Device 0's phoenix miner is always the one stuck?