Garmin 496 loss of satellite reception

Three times in the last two weeks, my 496 has lost satellite reception upon
descent below 2000' AGL.

This occurred in three different locations:

The first time was about a week ago, crossing the western shoreline of
Galveston Bay, just south of Ellington field. Moving the amplified antenna
around the glare shield did not help, so I replaced it with the stub antenna
and reception was restored. I presumed the remote antenna was defective,
and used the stub the rest of the trip.

Saturday, I checked the amplified antenna at startup and it seemed fine, so
I decided to use it and monitor its performance. It had strong reception on
at least 10 sat's until I descended for the visual approach to Slidell, LA.
At 1500' I heard the "Satellite reception lost!" warning and saw that all
the bars were depressed to near nothing. A few came back enough to
reestablish position, and the GPS remained "on line" until landing.

The third time was the trip back from Slidell to drop off the Angel Flight
folks at Fairhope, AL. Same scenario: descended below 2000' and satellite
reception was briefly lost.

I plan another trip this coming weekend just to see how repeatable this
problem is, but right now, I'm mystified. Anyone have an idea what could be
causing this? Pressure effect on the antenna housing, perhaps?

--
Dan
C172RG at BFM