I'm using an EVGA SuperNova 750W G2 so I don't think it's the psu.
I'm at 0.0133%/0.0138% error rate currently with 49C/51C temperatures.
check your psu very carefully . this one
http://www.newegg.com/Product/Product.aspx?Item=N82E16817438027&cm_re=evga_supernova_750-_-17-438-027-_-Producthas separate rails if you plug it in wrong you could be straining it.
this one has 1 rail
http://www.newegg.com/Product/Product.aspx?Item=N82E16817438017&cm_re=evga_supernova_750-_-17-438-017-_-Productbut the rail rates for 744.8 watts. if you run the s-5 at stock it takes about 610 watts. which is 80% of this psu capacity.
I think your error increase is due to this psu. it is just about maxed for 24/7/365 use.
humor me and set freq at 337.5 or 343.75m ..
run for a few days at 343.75 if errors stay flat lined then it is the better setting for the psu and the miner.
a review for your psu
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=380if you look at cold test 600 watts it is at 12.06 volts
if you look at cold test 748 watts it is at 12.03 volts
if you look at hot test 600 watts it is at 12.05 volts his hot test is a few hours and there is some sag.
and his hot test at 748 watts is at 12.02 volts
my guess is you are very close to max on the psu. and after days of running your psu is a bit too tired.
it sagged from 12.06 to 12.05 in 2 or 3 hour hot test at 600 watts
it sagged from 12.03 to 12.02 in a 2 or 3 hours hot test at 748 watts
rollback on the freq 343 or 337 and test for hw's
or keep it at freq and see if the hw's stop increasing. (they will if the psu is just past it limits for 24/7/365)
I have seen this happen and take 20 or 30 or 40 days to burn the psu.