External Monitors Losing Connection Mac Pro

I've seen this issue discussed with people using MacBooks, but not with Mac Pro. And the solutions offered there don't work for me.

System Setup

2013 Mac Pro (trashcan) w/ 3 Apple thunderbolt displays (yes those older ones). The system has worked very well until now.

Problem

Sometime after Mojave update (about a month ago) one and sometimes two of the monitors just goes away. That means it won't come on and the Mac won't detect it. At first I only noticed it happening when the Mac would wake from sleep. Two monitors would come alive and the third would stay dark. But now it will happen when I'm working. The monitors will flicker and the most problematic one will display pixelated boxes in many colors. Sometimes the system will recover after 5 seconds; it will flicker the monitors and I'll be fine for a while longer. This will happen 2-4 times and eventually - usually 30 minutes to 3 hours - the flickering will happen and the monitor won't come back on. Then I'll be running with the other two. Sometimes when I come back to the machine the next day and wake it from sleep one of the other two monitors is off and then I'm down to one.

What I've Tried So Far

Unplugging the monitors from the thunderbolt connection and replugging it in doesn't work. Nothing happens. Same when I unplug and plug the monitor power back in. Also switching thunderbolt ports doesn't prevent the issue nor does it make the Mac see the other monitor. A reboot always fixes the issue temporarily.

Tried reset PRAM and SMC - no change.

What I cannot figure out is if this is a problem with the monitor, the Mac Pro itself, or some driver issue in Mojave. I poked around in the console but I am not familiar with it and couldn't find anything.

Any suggestion on where to look in the console or otherwise poke around to at least narrow down the issue would be most appreciated.

Thanks in advance