Same here also very big variance in the reward ...One week ago all my reward was 0.0015-0.0020 now 0.0010-0.0015 ...
exactly 1 week ago the difficulty changed from 11,187,257 to 12,153,412 too
If there aren't any issues I have +-0.01-0.015 variance.
I haven't noticed any lesser payouts when the Diff goes up. most likely because it goes up for everyone so the payout should not be effected assuming you arent CPU mining or single card low end mining I guess. but when the pool jumped up by 1,000gh/s I noticed lower payouts negating one of my gpu. so I added 2 more.