Problem with FM tuner in Sony CDX-C680

I have a Sony in dash AM/FM CD player, model CDX-C680. This is
a fairly expensive item, setting me back about $350. It is installed
in the dash of my 93 Chevrolet G20 custom van. When playing CDs, it
produces a very nice sound, and I am quite pleased. However, the FM
tuner is causing me problems. I installed this about a month ago, and
at first I thought everything was great. Then I started noticing a
"pumping" effect when listening to music from the FM tuner. After a
great deal of critical listening, trying to determine exactly what was
happening, it seems that the high frequencies are "coming and going"
(increasing and decreasing) as I drive down the road. This produces a
"pumping" effect, much like bad Dolby Pro-Logic decoding, and is quite
distracting when you are attempting to enjoy the music. Anyone have
experience with this problem or anything similar? Should I attempt to
get this unit replaced under warranty?