Orange Peel finish is usually caused by the paint drying too quickly and not having enough time to level out properly. Acrylics are notorious for this because they already dry so fast.
The trick is usually to keep the paint from drying in the air between the airbrush and model and to give it ample time to level out before the carrier evaporates. You can do this in several methods:
1) Thin your paint a little more (although 60/40 sounds fine to me)
2) Spray a little closer to the surface
3) Spray slightly heavier coats
4) Lower your pressure (although the pressure you are using sounds fine to me)
5) Try a different thinner (although with Tamiya gloss paint their thinner is the way to go)
6) Add a couple of drops of acrylic retarder to the mix (although I think Tamiya thinner already has some retarder in it)
7) Keep drafts such as fans from blowing on the paint while it's drying.
Temperature and humidity also have a big effect on paint drying time, but there is usually very little we can do about that.