both cards are sapphire, increasing the memory + or - by even 5mhz results in lower kh/s with the 7950, similar problem with the 7970, intensity 13 is the sweet spot with the settings im running at the moment.
Which Sapphire model is your 7950? I have a Vapor-X that seems to behave similarly to what you describe. If I increase the engine or memory clock speeds the hash rate goes down. Here's what gets me ~610Kh/s:
"auto-fan" : true,
"auto-gpu" : true,
"temp-cutoff" : "84",
"temp-overheat" : "81",
"temp-target" : "66",
"gpu-fan" : "0-100",
"intensity" : "20",
"vectors" : "1",
"worksize" : "256",
"lookup-gap" : "2",
"thread-concurrency" : "40960",
"shaders" : "0",
"gpu-dyninterval" : "7",
"gpu-platform" : "0",
"gpu-threads" : "1",
"gpu-engine" : "1050",
"gpu-memclock" : "1375",
"gpu-powertune" : "20",
I set the temp target so low to keep the fans spinning fast enough to cool the VRMs, which have a separate heatsink on the Sapphire 7950 Vapor-X. Since the VRM heatsink isn't connected to the core heatsink, even if the core is cool enough, the VRMs may be over temperature. If that happens, I've noticed a sharp decrease in hash rate.