Mac Pro late 2013 Display Issue (Display Port + HDMI)

Hello!

Just encountered an issue (since couple of days) where one of my two monitors won't start (no signal).

Mac Pro (Late 2013) :

MacPro6,1

3,5 GHz 6-Core Intel Xeon E5

Dual AMD FirePro D300 2048 MB

64 GB 1866 MHz DDR3 ECC

Monitors :

EIZO CG277 (DisplayPort)

EIZO CG222 (HDMI)

OS X El Capitan 10.11.5 (15F34)

I've had this setup for quite over a year already, never had an issue until recently when I started the Mac pro and the HDMI monitor had no signal, unplug and plug back in the HDMI cable while the OS X still running would somehow stop the graphic card to send signal to the Display Port as well, rendering both monitor without any signal - had to hot shut down ... very frustrating.

Tried many things (different HDMI cable / DisplayPort to HDMI adopter / connect only either of the monitors) to resolve it, nothing seemed to help except shutting it dow, unplug everything (including power cord / reset SMC) and then power on with both HDMI and DisplayPort connected.

Problem is that once I shut down or restart the computer, the HDMI monitor will have no signal again.

Did I miss something ?

Please help!

Thank you.