I know this is an old thread, so you probably don't have that problem anymore, but I had a similar problem, which I fixed by making sure the computer with fluctuating/varying hashrate had enough power from the outlet.  I had two 1250w psu 4gpu miners plugged into one surge protector off a 15 amp rated outlet, and the hashrate went up and down on one of the miners. 

Then the power strip tripped, so I replaced the 15 amp outlet with a 20 amp outlet (20 amps from the breaker) and plugged each computer into its own power strip.  problem solved, hashrate stable.