Your trans. is too full and/or too hot. Is the fluid foamy? Overfilling a transmission can raise the fluid level to the point where the internal moving components i.e planetary gear sets, imput and output shafts, etc. are physically immersed in the oil, which "churns" up the oil causing it to foam, and be pushed out of the case vent, filler tube, etc. The foamy fluid won't dissapate heat as well.