Are you doing the air testing in your house ?? or out side ??
Could be that you are picking up some EMI.
I got to a site last week with my MXT Pro and 5.3 coil, with the coil in the air, when I turned the MXT on, it went into over load, never had this happen with any MXT that I have ever owned....I had to hunt with another unit with noise cancel...
Later, thinking, I would have to send in my new Pro, I drove a block away and the MXT Pro was back to normal.
The next time, 2 days later, back at that same site, the MXT was fine.
The only thing that I could see was, the school was not having a football game, so no big lights, no PA system.
And I was more than 150 yards away from the football field.
So EMI can bother any unit...
HH....BJ