Hey John, I can't really say anything that UPD above hasn't already said.
I'd start by stripping the whole airbrush down, do this over an old white towel or T shirt, that way when you drop the tiny nozzle it will stay on the white rather than bouncing onto the garage floor... Make sure everything is clear by looking through it.
Make sure you've either got a manual or have downloaded the exploded parts list from
airbrushes dot com
Check all of your seals are in place and intact ! This can cause blow backs with bubbles in your paint cup.
To start with I would mix the paint about 2:1 which should give you a good consistency and maybe have the pressure up to 35-40 psi, a little higher than suggested above and then reduce down from there.
With a 0.5mm nozzle you shouldn't really have any problem spraying it.
Hope it works out, post up what happens on here, it will be useful for other painters that suffer from any similar problems.