Don
I see you have solved the issue but I'm still working on the real reason. All I can say for now is what I know and think but I PM Randy for you to see what he says.
Noramlly, if you mix up a sulfate and choride mag sup there may be a precip if you fully dissolve both ingredients separately. That precipitate is calcium sulfate. Ther Ca++ comes from impurities in the chlroide and sulfate. The solution is to do what you have done shake it, although I like the IV/balloon trick. It may be that CO2 is leaving solution and getting trapped in the upper air space, raising the pH of the solution, thus precip. When mixed/shaked the CO2 brings it back into soltuion or the solution is just to concentrated. If you dissolved enough table salt in a jug the same thing will happen. Therefore, diluting it my work. It also may have something to do with temp or temp shift from when first mixed.