Ok. So now I have more information.
My has double graphics cards. One integrated Intel Iris Pro and One Nvidia GT 750M.
When I switch to Intel Iris Pro it works!!!
When I switch it back to Nvidia GT 750M problem is back.
(my imac has a Nvidia GTX 775M)
So maybe there is a problem with graphics drivers for Nvidia GT 750M and GTX 775M in Yosemite?
Info on Nvidia:
Chipset Model: NVIDIA GeForce GT 750M
Type: GPU
Bus: PCIe
PCIe Lane Width: x8
VRAM (Total): 2048 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x0fe9
Revision ID: 0x00a2
ROM Revision: 3776
gMux Version: 4.0.8 [3.2.8]