I had a lot of WindowServer crashes with my 30" external monitor at work when I upgraded to Yosemite. Found huge discussion threads on it on Apple's forums - it's been fixed in 10.10.2, so if you get on the beta channel for updates that should hopefully solve it for you. It fixed it for me and the latest beta seems solid. It was happening to me every time I unlocked my computer, and I would lose all my windows/tabs - maddening.