lockout torque convertor

I have a 1992 Buick Century with a 3300 engine with 155,000 miles. Above 45 mph it sometimes starts to hesitate like the lockout torque convertor is being told to lock then unlock. Usually this hesitation occurs when going up a slight grade with slight to moderate acceleration. We have replaced and/or repaired/cleaned several components, the fuel filter, plugs, plug wires, cleaned injectors, changed transmission fluid...but none of these have made even the slightest difference. Does it sound like the lockout torque convertor and if so, what's the repair proceedure on this?

Thanks,