Garmin 396 Reception Problems - Latest Observations

In my previous posts, I described what appeared to be random signal lock
losses with my new 396. Changing everything did not help. I got the
dealer to swap the unit with a new one and I upgraded the software to
the latest level.

It appeared to work well for over 2 months, however, it started fritzing
out again the other day.

This time, moving the GPS antenna DID make dramatic differences. So, I
was content on knowing that shifting the antenna back a few inches when
it went out to lunch would make it spring back to life.

However, I had read that certain nav/com freqs (even just receiving)
would give the thing fits. I already knew that constantly transmitting
on 123.0 would hack the GPS signal to zero after 6-8 seconds of
transmitting. But today, I discovered that simply tuning to 109.5 to
view one of the the local runway's ILS would almost immediately lose all
sats. Zero. As soon as I switched to some other freq, bingo. Immediate
solid reception. My #1 Nav/Com is a Narco Mk12D. Others have reported
these radios giving certain GPS receivers fits. It tried my #2 Narco
(Nav 122) and it did not appear to affect the GPS that much if at all.
Both navs receive from the same antenna with a splitter on it.

So now what? Some suggested that the Narco nav/coms were not properly
shielded. While others speculated that the interference is coming
straight from the antennas for the nav/com. How do I shield the box in
the stack or the GPS antenna to see if it makes a difference? What
material(s) will work?

Thanks,
Mike