Why does DNS resolution fail for one user account every 10 minutes on Yosemite?
apparently a bug with yosemite... I had the same problem on my son's account... no ping while dig or nslookup were working fine...
I have updated my mac with 10.10.1  (beta version available for developer) and everything is now back to normal..http://www.macrumors.com/2014/11/10/yosemite-10-10-1-build-14b23/