Could be a number of things .... but the first that comes to my mind is moisture getting in your lines.
Do you have a moisture trap on your compressor .... if not you probably should get one, they're fairly cheap at any major hardware store.
To adjust your pressure, you need a regulator. If your compressor doesn't have one, once again, check out your local hardware store and you'll find one.
Happy Modelling and God Bless
Robert