[LAU] Scarlett 3rd gen glitches (again) + Raspi 4

Hi list, I have a Focusrite Scarlett 3rd Gen interface that is producing audio glitches with a Raspberry Pi 4 running a realtime kernel (or not). I had similar problems on a laptop and on a desktop. On the laptop however it now runs perfectly with the very latest Linux Mint and the standard kernel. On my desktop I get acceptable results with AV Linux although haven't tried it with the latest Mint. There are no error messages reported or overruns, just glitches which are independent of buffer sizes and sample rates. The kernel installation procedure that I used for the Pi 4 is here: https://lemariva.com/blog/2019/09/raspberry-pi-4b-preempt-rt-kernel-419y-pe… and the actual kernel that I built is: 4.19.71-rt24-v7l+ Can suggest some things to try? Thanks a lot. Iain