I am leaning towards the devpool issue, do you also have AwesomeMiner software running? There could be a random rule causing that.

Reach out to Patrike or post on his thread https://bitcointalk.org/index.php?topic=676942.0 he would be able to help you better if you provide the API logs to some of the miners you have.

But to save time, change one of the miners' firmware to BO+ or Bitmain stock, if it runs fine then you know it's firmware related issue, also make sure your router firewall isn't blocking the dev pool.