This is not easy, but I'm going to play this one straight.
Marc you simply need to buy a bottle of pneumatic tool oil (Lowe's, Home Depot, Etc...) and put about 5 drops into the nipple that you attach the air hose to. Just hold the tool upright and squeeze gently, or else you'll put too much oil in. The oil just helps to keep the moisture in the compressed air from damaging the tool. In your case you probably can oil the tool every other use, unless you go for an extended period of time without using the gun.
I'll look at the adjustment on the gun next time I'm up.
I empty my compressors tank once a week. And Ryan is absolutely correct about moisture being the issue.