Strange Comm problem.

We are currently working on two new turbine aircraft built about 60
serial numbers apart. They have each presented the same problem.
Comm reception on frequency 118.5 deterioates after an hour so and
becomes unreadable when the tower is broadcasting. However, you can
still hear other aircraft responding to the tower on the frequency
without any distortion.

These are all glass cockpits with integrated avionics suites so as I
understand it there are no separate boxes to be swapped around. ( I
don't get to work on these yet.)

We have not been able to duplicate the problem on the ground or in the
air today after a half hour flight.

I am no avionics guru so I will ask a couple of questions. I assume
that the towers transmitter may be more powerful than those in the
aircraft. If that is true is there circuitry in the aircraft's
reciever designed to control or step down the strength of the signal
once it is received? Could a failure in that circuitry produce the
problem seen?

Could something like poor termination crimps in the antenna coax or
other parts of the radio harness have this kind of effect on only one
frequency?

John Dupre'