Hi Susan,
I don't know about anyone else, but I am extremely allergic to house dust.
In my house, it seems to be everywhere, and the only time I noticed that I
didn't have a frog in my throat was when I went camping outdoors and didn't
sleep in the tents...it was strange to wake up the next morning without
phlegm.  And the frog in the throat is only a constant symptom.  Whenever I
move furniture around or clean out a closet, I have to wear a hanky over my
mouth and nose, or else I start sneezing convulsively and my face becomes red
and extremely itchy.
I'm a singer, and frog in the throat really is a curse...cutting out dairy
has helped my nasality immensely but it's still there.  If you ever get the
chance, have yourself tested for allergies.  At least having an allergy test
(they test dust, pet dander, pollens, etc) will make you aware of things that
perhaps you were not aware of.
Again, I'm no authority on this in any way, but it's a suggestion.  It could
easily be pesticides, grain-fed meat, pollution, as you said...it could also
be just the architecture of your larynx that just doesn't allow for the plegm
to flow without obstruction.

sara