I think it could be the infamous Yosemite / CUDA bug…
I had this… My graphic board was replaced twice and still had the problem… The only solution was to completely uninstall CUDA and use OpenCL… Then, when the project I was working on was finished, I downgraded to Mavericks…
I don’t know if some CUDA update solves the problem but if you google “Premiere, Yosemite and CUDA” you find a lot of people having problems…