Radio Glitching
Chuck,
Something off the top of my head, maybe on this particular airplane the lengths of the wire are just right to resonate at 50 MHZ and cause the problem.
I have also run metal pull pulls with no problem and have used metal pushrods also with no problem.
It just could be that your lengths are ideal and are resonating.
You might try breaking up the lengths with insulators, like the kind that they use on antenna guy wires to break up the lengths. (much smaller of course) LOL
Good Luck
Jerry