I Have A Chronic Problem Of Congestion In My Throat.

I have a chronic problem of congestion in my throat. It 's associated with some post nasal drip. It's particularly troublesome at night and interferes with sleep. It began in mid Feb. and has persisted to varying degrees for about 3 months now. Decongestants help but also interfere with sleep. My doctor diagnosed at first that this was a virus but now thinks it may be allergy. I've not heard of allergy here in Pennsylvania in Feb. and a virus persisting for 3 months seems unlikely to me. Any suggestions?