Had been having issues with hashrates and high rates of rejected shares on a number of pools.  Reduced the freq to 800mhs, added #xnsub to the pool address -- has reduced the error rate significantly. 

xnsub stands for extra-nonce subscription. For algorithms with very high difficulties and for miners with very high hashrates, it's possible for a miner to completely exhaust the nonce-space (that is, the value that is changed for each hash to try and find the valid hash).

Extranonce was created to allow the miners to change a small field in the coinbase to continue hashing again from the beginning, but with a slightly different block.

By subscribing you're telling NH (or any pool that supports it) that even though you've already submitted a certain nonce, it's actually a different block because of this extranonce field.