After update to "OS X Update Combined version 10.9.5", my 4k monitor change to running at 3840x2160@30Hz. How can I return to a true 1920x1080@60Hz? Previous to the update i was able to change between the two resolutions.

Hi, my MacPro6,1 3.5GHz 6-core, FirePro D500 was updated to "OS X Update Combined version 10.9.5". After the update my 4k monitor change to running at 3840x2160@30Hz. How can I return to a true 1920x1080@60Hz resolution?

Previous to the update i was able to change between the two resolutions. Now it is always at 3840x2160@30Hz.

When i try to change the resolution, the text, icons and objects are scaled to simulated a resolution of 1920x1080 but

in reality the system continue to run at 3840x2160 @ 30Hz.

That is a problem, because it is always at 30Hz.

PS: 3840 / 2 = 1920 3840 / 3 = 1280

2160 / 2 = 1080 2160 / 3 = 720

Previously, I was also able to use 1280x720@60Hz from time to time. Both run as natural resolutions without artifacts.

How can I tell OS X 10.9.5 to change to a true new resolution? as before (I believe it was 10.9.3, not sure).