Thanks to all for their comments. I should have mentioned that I did do a couple of full resets at the time, with no change, I'm afraid.
Dave's observation of the audio muting in the presence of strong signals was interesting, as I was staying less than a mile away from a 30kw transmitter before and after this part of the trip. But, that wasn't the place that I was trying to use the radio.
And, this Skywave must have been strengthened by its baptism of fire, because I've just taken it to within a mile (all salt water path) of my local 10kw, CFAX-1070, within its very directional lobe (nearly 2V/m field strength at 1 km supposedly). It was happy to continue working on the AM band in that environment.
So, I'm back to its front end getting locked up by something else. Best to take Gary's advice about taking it in the carry-on, it would appear.