I'm having a touch of trouble with some of my cupcakes lately.  It doesn't happen every time and it doesn't happen with every recipe (it's not even consistent within a batch) but some of the time when I go to de-pan my cupcakes, the bottom of the cupcake is concave instead of flat.  

Has anyone else ever had this happen?  I'm trying to determine what is causing it as the wasting of cupcakes is not something I am fond of.  I recently purchased a used Deluxe CR-2-4 and I bake at 350 with the foil liners from CalJava for everything.  Any thoughts?

Thanks!