nsz,
I don't know what the cause of your problem is, but don't downplay the sinus angle. I found out I had a really bad sinus infection and the doctor said I had a serious allergy problem. That was shocking to me as I had no clue I had allergy problems. The doctor said it could be related to anything, not just from what's outside. It could be from what's inside your home or something else.
If it's sinus related, I use a neti-pot and rinse my nasal cavity with it. It includes packets of ph balanced isotonic mixtures that clean out the sinuses. It's really helped my sinus infections and tamed down my allergies. I have far less mucous in the mornings like I used to. Might be worth checking out http://www.neilmed.com/usa/nasaflo.php
I bought mine at Walgreens. Amazon has 66 user ratings at almost five stars for this product. Amazing how well it works for me!
Gary