[smokeydan]

Success!
I've just been through the plug in folder, updating this and that. I discovered that certain Waves v9 plug ins do not play well with ProTools 2019.12, especially Abbey Rd Plates and Kramer Tape. These were both causing spikes in the CPU meter and then a variety of errors. I updated all waves to v11 and got rid of the v9 and v10 waveshells, and now it can happily run all of our multi track sessions at the minimum buffer setting with stable CPU usage showing (far lower than on PT2018) with just one exception - if an instance of Altiverb is active on an audio track, it will not allow uninterrupted playback on the minimum buffer setting. In this instance, to play back without errors the buffer size must be raised to the highest setting to allow reliable playback. Multiple altiverbs on multiple aux channels is not an issue.

So, apart from everything else that's going on right now, I'm delighted. I've been wrestling with these issues for two years! I might have a drink to celebrate.