The same was with me, lol.
I played with dual mining ZEC/ETH and added -i 1 option to claymore's config.txt.
So after switching to ETH the performance was VERY low on 2 rigs of 3. I even reinstalled windos. Lol.
Ok ! Good/Bad to hear that i am not alone having this issue.
I have had all 480 Gear on -i 2 @ZEC. Not my 470 rig and that one is working as it should on ETH.
Now i am back mining ZEC due the little bump upwards in price and all rigs run att full speed.
What seem to suck is that even if i reinstall all my rigs and get full speed @ ETH again i will probably have the same result again as soon as i switch to ZEC mining again.
So the best would be to find a solution for this problem.
I will hazzle around a bit as soon as i get some free time.