Transmission problem for grand prix experts

My 08 pontiac grand prix base model has a transmission or torque converter surge or slip issue. When taking off, it's fine until ur speed is fast enough for torque converter to start locking up. Then it feels exactly like the torque converter is locking in/out/in/out rapidly-  like 2 in/out cycles per second. Push on the throttle a bit and it quits slipping /surging until you let off. Other conditions would be like on interstate in high gear when you come to a hill and go on the throttle a little more. Then it starts it's surging again. Mechanic shop replaced and programed the Ecm, cuz they said the computer was telling the Torque Converter to lock in/out. That didn't fix it. If you unhook the battery an reconnect, it will fix itself for a few hundred miles. Trans shop wants to rebuild the trans, but I'm afraid I'm gonna rebuild trans and the Ecm will still be screwing with the tcc thus burnin up the rebuild. Why would Ecm be acting so strange? All the engine problems causing check engine light have been fixed an still didn't fix it. Trans did get very hot and slipped terrible on a hill a year ago. But ran fine since. Yes flushed the fluid. I put 20,000 miles on the car with it acting like this and it isn't getting worse. I drive it decently though. But, on a hard take off, it shifts great... probably cuz torque converter isn't trying to lock up. Wisdom please. Yes I know it needs rebuilt, but not sure it's gonna fix it totally. It has 125k miles.