Optiplex 5070 optional HDMI Connection

Hi, 

We have about 10 Optiplex 5070 SFF's with the additional optional HDMI port.

All of which are connected to two P1914S monitors (via DisplayPort) and one P2419H (via the HDMI).

This has been happening for about a year now but have only just discovered how/ getting around to asking the community;

On some of them (4/5 of them regularly about once a week) the HDMI port will disconnect and the port/screen will become unrecognisable. It tends to only happen when a user tries to log back in after locking their screen to go to the kitchen for a coffee for example.  

I've tried various other monitors and cables along with various solutions from the community that seem to be for similar symptoms but nothing has worked so far. The only temporary fix seems to be a cold boot.  

Any thoughts?

Thanks 

Bob