Sounds like what is happening to me in my old age.
Seriously, your problem does sound like paint senility. No model paint is stable forever - at some point the pigments, carriers, etc congeal and it begins to look like jello. In my experience different paints decay in different ways. Paints seem to vary widely in shelf life. I have some paint that is fifteen years old and still mixes beautifully. On the other hand, I went through my paint collection just before a recent move and I bet I threw away 30 bottles ![Shock [:O]](/FSM/CS/emoticons/icon_smile_shock.gif)
The majority of it was MM enamel.
The real enemies of paint in my experience are heat, exposure to the air, and evaporation of the solvent. Usually it is a combination of the three. The best thing you can do for paint is keep it cool and keep it tightly sealed and unagitated. I always wipe the rim of the bottle after I open it to remove any paint that might hurt the seal. I also put stretch wrap over the top and screw the cap over it. I don't know if this actually helps or if it just makes me feel like I am doing something.
One final tip - always buy the bottle at the hobby shop with the least dust on it. I suspect some of the paints I have purchased have been on the shelf a LONG time. It doesn't help of the store gives the paint a 5 year head start on its shelf life. Also check to see if the lid is tight or if any of the solvent seems to have evaporated. A surprising number of bottles in the store have fairly loose lids and seem to be a bit down on solvent. That's a very bad sign.