rickyinc63,
I don't have much airbrush experience, but what I have done is with an Aztek and mostly with their acrylics. I sprayed it as it came in the bottle. It sounds like you are spraying too close and/or with too much volume of paint, just too much paint hitting the model at once. Try backing off and think in terms of making passes of lighter coats, not to try to have the finished look with one sweep of the airbrush.
When I saw some runniness start and backed off and as I described worked. Having said that, you can also get too far away and the paint starts to dry before hitting the surface, resutling in a rough or grainy surface. There's an inbetween but you should find it not hard to find.
Thoughts anyone else?