It was a 2.5A charger - but it was from a Cana kit - and plenty of people seemed to have issues with those.  I have a nice reliable 2.1A tablet charger on it now.  12 hours in and not a single hiccup.

I did add a script to check the ethernet connection periodically and reboot if it drops because that seems to precede the lockup.

So far - so good. 

Thanks for the responses fanatic26.