Hairworks enabled?
Seems like overclocking my gpu's was the culprit, haven't crashed since I went back to the default clocks in afterburner. If I turn hairworks off it never drops below 60, with hairworks I get some drops to 55 and more stuttering. SLI 970/2600k @4.4
Really wish you could turn it off for Geralt and leave it on for the monsters.