Enough theory allready.
Time to apply moron math to the problem. He is the guy that cuts lumber by holding the piece up to the gap and marking it instead of doing mental gymnastics.
The first step is to connect the radio directly to a head set with the headset adapter that comes with the radio. If that fixes the problem it is the intercom not the radio. A known good headset is another thing to try.
The second step is to take the known good setup, the vxa 150 and try it in place of the vxa 220. If it works the antenna, powerful radio stations, engine electrical are not the problem.
Third step is optional. Move the VXA 220 to the plane where the vxa 150 is. If it doesn't work then start troubleshooting the vxa 220 and intercom interface.
Always try the easiest cheapest thing first. And be methodical.
Hope this helps.