Tony you have had your fair share of problems with this paint and thats a bummer given all you have done. I have painted alot in my life and ran across mixing problems with some paints. What happens is the different weight pigments in the mix settle in the bottom of can in the order of weight and the longer it sets there the firmer it gets. If you drag a stir stick across the bottom of the can and lift it out you will see what I mean. Shaking alone will not free it up alone, it takes a stick to get the pigment lose from bottom of can or an impeller mixer on a drill will churn it up pretty good also giving you a good mix before the reducer is added and then finish up the mix with the impeller. If the paint sits more than 10 min. between the times you put more in the gun pot it needs to be stired again, the different pigments will start to seperate again. Shaking can cause another problem, airating the paint (filling it with tiny bubbles) aspecially two part urethane colors and clear coats, those paints should never be shake mixed, impeller or stick mix is the propper way to mix those paints.
Automotive paints are not typically shake mixed, they are stired, and thats about what you have and possibably why you are having those problems.
Leroy