Display freeze for 15 seconds in Mojave

I've seen this several times since I moved to Mojave - often enough that I thought I would report it here and see if anyone else is seeing this.

Occasionally - maybe once every couple of days on a machine in fairly constant use - my MacBook Pro 15 (late 2016) running Mojave will get itself into a strange state where it starts to freeze.

Initially I could not see a pattern to this, but it's now very clear. When the problem occurs, the whole platform freezes for 15 seconds whenever I change to a new application window. It's always 15 seconds exactly and it's always when changing between windows.

For example, I'm running an app and the problem starts to occur. If I click on another app window to bring it to the top of the window stack the system will immediately freeze after fronting the window. After 15 seconds everything resumes as normal. If I stay in that new top window everything is fine. But as soon as I switch to back to the first app - front it's window - the problem occurs. If I leave the first apps window in the background and send scroll events to it - all OK - or if I right click over that window - all OK. So long as the window of the app I'm running stays topmost I'm OK. Switch to another app window - 15 seconds of freeze - then resumption.

This is extremely repeatable - the behaviour never changes. It does this in any combination of windows. Most of the time Mojave is fine - but occasionally this behaviour starts. The only way to stop it is to save work and reboot the OS. I have not yet found a trigger - it seems to be (thankfully) infrequent and random, but frequent enough to be a real pain.

I'm running in clamshell mode connected to a Thunderbolt Display 27 though a TB3 to TB2 converter most of the time. I have not yet seen this behaviour on the laptop alone but that's because I don't use the laptop on it's own for long periods of time at the moment. So it could be related to an external monitor. If I see the issue occur when I'm just running on the laptop I will update this thread.

I've seen other reports about strange graphics related issues that have, like mine, started occurring only with Mojave.....

Anyone else seeing this?