You DON'T want to add any capacitors to the P leads!

Have you adjusted the gain, squelch and mike gain (I think that's the
nomenclature) on your I-Com radio? The I-com I have is a very
sensitive radio. You just may have the squelch incorrectly set (I. e.
too sensitive) for your installation. You have to remove the radio
from the tray to get access to these 3 controls.

A key point - do you get mag noise overriding on normal radio receive
signals? or just when the radio breaks squelch? That would tell you if
the mags etc are radiating excessive noise.