When I set the screen to Large (thanks to that excellent toolbox64), and use RealVNC, I find that FLDIGI sometimes freezes the SBITX waterfall display, particularly when changing modes.
I've tried turning down the waterfall speed with the latest DREXJJ version.
I wonder if there's any conflict between RealVNC and the audio drivers on PI. ....shouldn't but I don't know.
I thought it might by my 4GB Pi, so I swapped it for a 8GB Pi, but same thing,