ntime out of range

[jjj092353]

I'm running stratum connected to mpos with new Rock Miners that when connected together are about 2 GH/s. for long periods of time I will repeatedly get :
[terracoin](Thread 1) Share rejected: {"job":"291","ip":"nn.nn.nn.nn","worker":"ninja.block1","difficulty":256,"error":"ntime out of range"}

it will eventually recover - this is an sha256 coin. MY stratum works fine for srcypt coins (I run many) and I have found that if I set : RollNtime[s] in the rock miner setup to a value like 390 it helps - If I set to the default of about 230 it never recovers - but eventually (within a few minutes) it goes back to repeated ntime out of range errors when rejecting shares

any ideas as to why? clues - things to change/modify?