I sounds like a simple case of electrical interference
from the vehicle ignition. Regarding the fluctuation
when under the bridge, I suspect the overall reception
is reduced, but the electrical interference remains the
same, giving the illusion of an increase.

Unfortunately, although I have heard of suppressors
to control this, I've no direct experience with it, other
that from the perspective of one having experienced
the problem.

Surprisingly, there is either not much on the net related
to this, or I was just inept in building the correct search.

As kids(aka teenager drivers), we simply started at the
ignition leads(aka spark plug wires), and replaced every
component until the interference was either tolerable or
disappeared. However, in today's vehicle, I would've
thought that would no longer be a problem.

Typically, a high performance type ignition lead would
significantly increase this interference. Or, possibly what
we thought were high performance were really just cheap
unshielded junk. Is it possible that your leads had been
replaced by something non-OEM?

However, I did find these:

. A reference to a Popular Electronics article of February
   1975(page 80) - "How to shield you auto ignition and
   distributor to prevent radio interference".

. A reference to a Skinned Knuckles article from January
   1994(page 32) - "Tip on curing auto radio interferen-
   ce by grounding the hood".

http://64.233.167.104/search?q=cache:Y5yPYHlRUasJ:www.morrisminoroc.co.uk/techni
caltips/Radio_Interference.html+%2B%22automobile%22+%2B%22radio+interference%22&
hl=en&ct=clnk&cd=8&gl=us

http://64.233.167.104/search?q=cache:5rQAoOXFlY0J:www.indiacar.com/infobank/car_
radio.htm+%2B%22automobile%22+%2B%22radio+interference%22&hl=en&ct=clnk&cd=12&gl
=us

The radio is not getting good reception or there is interference from an
electrical source in or near the vehicle.

Check to make sure the antenna connections are secure and that the antennas
are not damaged.  I think that your RX has two antennas - the mast antenna
and another one embedded in the glass in the right rear window or rear hatch
class.

If the interference is related to engine RPM and you have had the spark
plugs and/or spark plug wires replaced, then use OEM ignition parts as
aftermarket parts sometimes cause the problem you are describing.