Sound cutting out

The sound is rapidly cutting out and coming back on playback - jittering - every few seconds or so.
I am using a 2018 iMac 16gig RAM. Dorico Pro 4
I have watched the video on adjusting the HALion RAM (to max) and core (to three) and I have played with the Device Control Panel buffer size. Nothing makes a difference. The problem sometimes goes away for a few minutes for no apparent reason then comes back.

Inputting notes is fine. The green playback marker is fine.

I have 6 flows in this project and three instruments - violin cello and piano.

The jittering sometimes gets worse when the instruments play fast notes. But not every time.
Please can you help

Does this happen also if you have no other app but Dorico open?
Could you please do from Dorico’s menu Help > Create Diagnostics Report and post the corresponding zip file here?
And one last question: Could you make a short audio recording of the jittering and dropping out? Something dirty and simple is sufficient, e.g. recording with a smart phone. I just want to get an impression of what it sounds like. Thanks