What you may have (not 100% certain) is a chronic fungal infection, which can cause the post-nasal drip without other sinusitis problems. A former Mayo Clinic researcher, Dr. Jens Ponikau, has a patent to find an anti-fungal spray, but it hasn't come to fruition yet.
The problem often lodges in the upper sinuses, where sprays and saline irrigation won't reach, due to gravity. A Flip-Turn Sinus Flush often works:
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground. Or, go to the comment I made on July 13, 2009 on how to do a sit-up method.
Note that, in between my severe bouts of sinusitis, before I developed the flush, I would go for months at a time with just a post-nasal drip, like yours. Nowadays, my wife has to call my name, as she can no longer track me in the house by my coughing.