You're right, this kind of fault is similar to a faulty antenna connection & that was
one of the first things that came to mind (and checked), but the fault was specific to or noted only with recent software
changes to SDR# and not happening on all freqs.
Youssef is like an Olympic sprinter at rapidly resolving these bugs. Fixed with release 1653 of SDR#.
Many thanks for your time PMM :-)