Transmission will "spit up" about 1/2 cup of transmission fluid

3 or 4 times a year, my transmission will "spit up" about 1/2 cup of transmission fluid. The repairman cannot find a leak and he says the transmission is operating as it should. The car has had the transmission fluid changed every 30,000 miles in accordance with the owner's manual. The "spitting up" of the fluid occurs after the vehicle has been sitting overnight. The last two times the problem occured, there seemed to be more transmission fluid ejected and the time between the fluid loss was closer. What might be the problem and what will it take to fix it?