06 Pontiac Grand Prix Transmission/Power Lost.

I have a 06 Pontiac Grand Prix it has 128,000miles i just had a rebuilt transmission put in it. But now when driving the car for about a hour when the car gets really good and hot. Its like the transmission is slipping. I took it back to the shop that put in the transmission and they said it`s` the C.Converter i took it to a muffler shop they did a back test on it and said it`s fine.(the converter that is) It makes my Traction Control light comes on. and it`s like the car does really wanna go. To me it really feels like the transmission is slipping cause it jumps alittle. but sometimes when you push the gas paddle it`s like the car wants to take off.The rpm hand jumps up a little every now and then when the car is messing up. But if i park it overnight or for a few hours and let it cool off. and go back and drive again it`s fine.. until it gets really hot again. The check engine light isn`t on. So what is wrong with it?’.