Hi Luigi,
I should do gardening as well, damn ;-)
I usually have between 30 and 60 seconds before the stuttering starts. Just enough time to record some riff ideas ;-)
When listening to recordings in Garageband, I also get the error. When using my Toneport as a "normal" soundcard e.g. for watching youtube, I do not get the issue.
No, I have not heard from Apple today. Support promised, that they will get in touch today or early next week. I hope, that the sound issue will be fixed with 10.15.5 which is currently in beta.
I don't think that Line6 will help solving the issue. Our hardware is old including the Toneport (at least in my case). This is an Apple problem. When you search the internet you find several posts of people who had or have the same issue in the last years. Sometimes it was fixed with the next update, sometimes not...
I now bought a USB SSD and installed 10.14. with my recording software on it. When I record, I start from USB now. With this method, I do not need to downgrade my Mac and have no change at all in the current installation. I do not have the disc space, because then you could also install a dual boot system on the internal HDD. It's not ideal, but I can work with it. Nevertheless I want Apple to fix their issue. Let's see how that works out.
Let's see... I'll let you know.
Ede