Mac Pro TrashCan 2013 Refuses to Use 2 Monitors

I have an upgraded Mac Pro TrashCan (2013) that REFUSES to connect to my second monitor. The second monitor is a HP 23cw that works for 10 minutes initially (to the point where I thought it worked perfectly), then flashes, becomes pixellated, and flashes the message "Input Signal Out of Range" even when I change to lower resolutions - I am unable to change to the specific one it states (1920 x 1080) even when I have show all resolutions ticked. I have the HDMI plugged in and the external monitor plugged into TB bus 1. I've tried using bus 2 and 0 to no avail. I can't get it to work or find any answers online.

I've Tried:

-Unplugging and Plugging back in

-Restarting the computer with the monitors both plugged in

-Changing Thunderbolt busses (1, 2, 0)

-Changing Monitor resolutions (all the way down to 480p)