I would also suspect that the problem is at the source. I am wondering about a relay clicking at certain intervals in the amplifier, but this has not caused the sound to interrupt so far. Does this happen with a hardwired source ?

This will not be related to the speakers. It is more likely to be related to the source.
The receiver also has its own DAB radio. I would test this with this radio. Provided, of course, that the DAB reception is sufficient.
Otherwise, test with an analogue source.