Same issue with my R9 280x which is a rebadged 7950, there are a lot of folks with this issue. Most likely its the AMD mac drivers in 10.10 all the way up to 10.10.4. On a positive note I installed the beta of El Capitan and the issue was gone.
Read the thread below and you will learn more about it and how to fix it for now running Quartz De Bug and disabling beam sync.
http://duc.avid.com/showthread.php?t=366123