Mono, I am all for stripping it back to basics with a frustrating problem like this and it's definately worth conclusively eliminating the coax...but bear in mind that he reported an identical problem for both COM1 and COM2...both coax's bad the same way?
Someone also suggested headset...hmm, it's common audio for both radio's...got to be worth eliminating the possibility that it's a fault related to the audio panel too ...though again I think the effect that changing the COM1 antenna had says that the radiator is getting in via the antenna.