That could be the problem, both the type of alcohol and alcohol itself. Tamiya's thinner is basically 91% iso, there's debate as to whether or not there is more in there, but a friend used the more common 70% iso and his Tamiya acrylics reacted a bit funky.
Have you sprayed it unthinned? It's worked fine for me that way. A guy on another forum said an airbrush 'guru' he talked to uses distilled water with his Acryl, about 70/30 paint/water mix.