[Elecraft] KPA500: strange behaviour - any ideas?

[Richard]

This happens to a friend almost daily during our evening rountdtable QSO 
on 160m...
Never on other bands. Since some seem to have cured the problem by 
changing antennas, my best guess is that this could be caused by RF in 
the shack.
Maybe a few ferrite cores on the coax shield or a coax choke could help?
Would be worth giving it a try.
In my case, it happened only once so far - on 80m - although I'm using a 
window line right into the shack to a balanced manual tuner.
YMMV

73
Richard - HB9ANM