Hi Andy,
You need to take care with these transponders that you always use a good quality grounded antenna. The final stage output transistors are not so forgiving and can burn out with a bad load. Also, I would't advocate having an assistant hand hold a transponder antenna given the microwave frequency you're working with.

Your idea of swapping transponders is a good one, but you might consider putting your transponder in someone else's ship with a known good setup. That would let you know pretty quick if your GTX320 is still healthy.

Another thought, you might want to kick this over to the Aeroelectric forum on the Matronics site. http://forums.matronics.com/viewforum.php?f=3. There's a lot of expertise over there.

-- Paul S