Hey Steve, sorry to hear you're having trouble. Solid freeze-ups were also a symptom Acidjack and I experienced on our two bad decks. I can't say I remember the last time it happened to my most recent deck, but don't doubt it's locking entirely.
That said (and to reiterate Kirk's troubleshooting steps), make sure you try to clear the din unlock message by pressing the front panel knob. If I get a din unlock message on my current setup, that is the only way to clear the error message.
When running the AD2k, I've had the most stable results by using an XLR > Coax 110 ohm digital cable and by setting the AD2K to AES output. IIRC, the 680 would not lock at all when using SPDIF at 96kHz using coax out. Actually, I can't remember if the coax vs. xlr output had anything to do with it, but I know for fact things got better once I selected AES output. I mostly prefer the xlr out simply because it locks
Are these freeze-ups happening randomly or as soon as you try and start recording?