What comes to mind first is have you checked the temps with an oven thermometer? Mine was off quite a bit and had to set it at about 310 to achieve 350.
Next is the amount of paint on the head. I wonder if too much paint is making it brittle. That happens with Gel coat on boats. Overheating the heads when coating may have too much paint for it to be flexible.
I know that you're no rookie at this so please don't take offense, but this is what went through my mind as I read your post. I haven't done any in a while and need to pour and paint some for myself. Regardless, I'll be paying attention to find out what you turn up as a solution.