Another cough suppressant you can try is Delsym.
Is there any family history of asthma? Coughing at night can be an indication of that. Also is the daytime coughing in response to anything - exercise or laughing, cold air or food - a popsicle or slushy will set me off if I eat it too fast. The humidity could set it off, and the generally nasty air conditions we've had outside.
Check the air filter on your a/c. My dh changes ours more frequently than recommended because of my asthma, and particularly if he notices me coughing or sniffling at all. He also uses some super hypo-allergenic type of filter. They do cost more, though.
Another "secret" home remedy for coughing - don't laugh - chocolate. I didn't believe it, but I swear it's true. DH brought me a bag of those chocolate stars when I had pneumonia, and it really did help. Temporarily, but it did help. I let it dissolve in my mouth, and I think it coats the throat or something. But it works.
Oh - warm tea with honey and lemon helps. Or just warm water with honey and lemon. Or, just honey and lemon. (My youngest is 9 now - I don't know what age they say honey or chocolate is okay now, but then they said 12 months)