If the fluid is not dark brown and acrid smelling, I'd say you have a torque converter lock up solenoid sticking. My best suggestion is to put seafoam, Lucas or trans-x trans additive and see if it frees itself up. If it doesn't within a couple hundred miles of driving, then repair is necesary. The part is cheap ($15-$20), but the labor is $$$$, as it requires the trans to be dropped for clearance.