4K Display problem with Samsung Monitor on Mac Pro

I'm having trouble with my 4K display on mu Mac Pro. I'm using the Samsung U28E590 display and a MiniDisplay Port to Display Port adapter. Recently, i see that my computer will not properly wake from sleep. I have been on Apple Support multiple times, and have reset RAMs, and tried new cables, but the fact of the matter is that the display "powers" up when awaking from sleep, but the picture never comes on. The only way I can fix it is by changing the wire from one thunderbolt port in the back to another. Then it powers up normally with the picture. Monitor works normally with HDMI, but the refresh rate cuts in half with it. So, has anyone else experienced this and/or found a fix?

Also I note that when booting up, the screen does not show the apple logo and the progress bar. It eventually comes on to the login screen. Sometimes it doesn't come on at all, and I have to change the thunderbolt port again.

This is my second Samsung monitor (different 4K versions). Same issue with both, so I believe its either a Mac issue, or a Samsung technology issue. The cable was switched as well without a remedy.

All apps are up to date, and running El Capitan. - latest version.

Thoughts??