Your paint mixture is partially drying in the air before it hits the model.

I had that problem with Tamiya as well, always too cheap to spring for the Tamiya Thinner, until one day I bit the bullet and bought it. Instantly my airbrushing was nice and smooth, no more drying on the way to the model, or worse, in the airbrush.

I had tried all kinds of combinations of water, alcohol, windsheild washer fluid, and different mixtures of all 3, and nothing worked as well as Tamiya's thinner.

You could try some retarder with your alcohol (plain water for thinner never seems to really break the paint down right) If you have a Michael's (or similar place) close by, pick up a bottle of Liquidex retarder to put with your paint mixture....or just pick up a bottle of Tamiya Acrylic thinner and use it to thin your paint.