How much difference in thickness is there between the rim and the bottom of the pot? If the rim is fairly thin and the bottom is thick, it could be retaining more heat and the thermal differences could cause the cracking. I cannot tell from the pictures - have you trimmed out part of the foot? If you have not tried it, that might help equalize the temperature variation as well.
I also agree with those saying to try a different clay body, or add some sand.