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.
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.
Although I stopped mining nearly 1month ago due to a 5970 quite litteraly going up in smoke, I can verify that this solution does work: I had placed a dedicated 3kw UPS onto my mining rig (It's good to be friends with people who own data centers
) and the cleaner power stabilised my hashing rate by a substantial amount.