MacPro 2010 / Ati Radeon 5770 / Mini Display Port

My system is composed of a mid-2010 MacPro with 32GB of RAM + ATI Radeon 5770 + DELL 2713H monitor connected through one of the Mini Display Port outputs.I have been using this system without a problem for more than 4 years, with different internal SSDs for OSX 10.6.8, 10.8.5 and 10.11.6.Although I had been using only El Capitan for some time, a few days ago I noticed some banding in the gradients.The native resolution of the monitor is 2560 x 1440 and I have never had this problem before, so I thought that the color depth would have been changed in the control panel. To my surprise, when I opened the control panel I noticed that in El Capitan there is no option to change the color depth.To see if it was a problem of El Capitan, I tried to boot from Mountain Lion and Snow Leopard, but while the system starts and works (I verified from another computer through shared screen), the screen remains black. Then, I tried connecting the monitor through DVI, verifying that the screen works, but when returning to the Display Port, the screen remains black.

Through DVI, 2560 x 1440 can not be obtained but the color depth is correct.This system and this monitor have always been connected through Mini Display Port working without problems at maximum resolution and depth of color with any OS. Now, the Display Port outputs only work with El Capitan and only at a reduced color depth, I guess 15 bit. I even ordered a new DP cable, but problem persists.

What could be the problem?Any suggestions or help will be appreciated.

Thanks,

CR