I'm experiencing similar issues with R9 290X. But after trying to reproduce it few times (crash in Witcher III, crash while running glmark2) I'm getting convinced that it's actually a temperature problem - the time it crashed with glmark2, the GPU reached 105°C. And now that I kept it under 95°C (by limiting the power to 100W), it haven't crashed during the few times I did run glmark2.