[Kenwood] mobile Kenwood TS-50S receive muting problem on SSB all bands

[Dale Castle]

Hello,

    This may be more of a problem with my mobile installation than the radio
itself, but I was hoping someone has run into this problem before.
    My TS-50S has two 4AWG running to both battery terminals with 30 AMP fuses
on both terminals. I'm using a 40 meter Ham Stick mag mount antenna.
    When I start my 30 minute commute to work in the morning, I can hear
stations and QRM clearly on 40 meters and make contacts. After about ten
minutes,(not always the same geographic location) the QRM disappears completely
and the stations are breaking up. I have to turn up the volume all the way to
hear anything. (Normally, I have it half way.) 
    The Kenwood tech support and another shop that works on Kenwoods want to
see
the radio, but I have the feeling that it's MY installation. 
    For example, I do not ground the RF ground to the chasis and am wondering
if
that is causing a problem.

    Any ideas are welcome! Previous owner did not have this problem, but did
not
run mobile either. The problem also happens after the antenna is removed.

Thanks in advance.

73s...Dale
KD4EAP