I just installed 1.31 on my router, and honestly, it's HORRIBLE!
I was using 1.21 before, with little to no issues, once I updated, I too am having this issue.
I WAS VERY SUCCESSFULLY using: north-america.pool.ntp.org
Since the instructions were to SAVE a copy of the Router configuration file, then flash with 1.31, THEN reset to DEFAULTS, THEN restore using saved configuration file -- I did just that.
HOWEVER, because I was using north-america.pool.ntp.org very happily before, no issues, and that was part of my saved configuration file, now because of this BUG, I have to go back and manually setup all of my fields, otherwise endless router reboots.