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.
i have the vapor X to, temps stay below 80c. my 7950 is housed in a CoolerMaster HAF case with 3x 250mm fans and 2x140mm fans. so I wouldn't expect my card is getting overly hot in there. the 7970 is completely in the open (free air) and stays pretty cool. Ill try your settings and report the results. thanks for help!