Okay, so today I spent the $$ on a 2L erlenmeyer flask from the LHBS. I didn't pay much attention to it, but noticed later that the bottom is concave. I thought they had flat bottoms, no? Anyway, it doesn't throw the stir bar. Neither does it spin. It is just held in place. I can hear the plate fan operating, but the bar isn't spinning, just sort of hovers in place. I guess the guy who built it used very small rare earth magnets. He thinks larger magnets will solve the problem. No wonder I like dry yeast!