The Createx airbrush acrylics should not behave that way (OP), I agree with the old/bad/expired totally. As to the consistency, this line doesn't start in the toothpaste range, but rather already in the milk-ish range. I use them especially for their metallic/pear/iridescent lines, those are just dreamy to use as long as they are kept well mixed, and no, I don't currently own an airbrush. ;-)
I will also totally second the Golden line, can't say anything on the Angelus because I haven't tried them personally, but I can say that I also like Liquitex for acrylics.