What is your ambient temp? Can you manage to get it a little bit warmer?
My ambient temperature should be around 15 ºC
But what do you mean by getting it warmer? Isn't it better to have lower ambient temperatures?
My you have a lot of reading to do
28nm seems quite peculiar in this matter... it seems that some miners the warmer they are the better they perform. some people said it could be related to GE VRMs rather than asic chips themeselfs, take this into account since your miner use Ericsson rather than GE.
anyway use the search function of the forum to get more info about it