UR12 crackles followed by sudden latency


I have the following problem with my UR12 almost since I bought it and can’t sort out how to fix it:
The interface very often sends a short crackle distorting my signal (sounds a bit like bit-crushed). Sometimes that brings with it a latency of up to 1s, which will not change unless the USB is disconnected or the DAW closed. It makes the UR12 completely unreliable for any live situation and also a horror for recording.
I have checked various input sources, cables, DAWs. The sampling-rate and bit-rate are synchronised to 44100/24. It happens with or without the Yamaha Steinberg application.
I’ve had very similar problems with another Steinberg UR12. I’m working with a Macbook Pro from 2014, El Capitan 10.11.6. The USB connections are otherwise fine. Any ideas?

Thanks in advance for your help