Hi, I solved all my video problems (random output signal between DisplayPort and DVI) installing clover as boot manager and activated UEFI in BIOS (Disable CSM). Now I've 2 GTX 690 working like a charm under Yosemite 10.10.3. Post me if need more detailed information.
I finally solved my problem, no more random video output. Now I have 2 gtx 690 (4 GPU) working without problems over Yosemite 10.10.3. The solution?, I disable CSM in BIOS and install Clover as boot manager.