Your problem is a 10 minute fix.
There is nothing wrong with your transmission.
The car uses a factory feature tune which detects and anticipates your driving style and adjusts parameters accordingly... mainly to do with the transmission. This is done for better fuel economy. So when you drive like a senior citizen the car detunes its self to save fuel. This has a direct effect on the way the transmission operates, shifts, and handles power. All of the automatics have this, v6 and GT.
Open your hood. Disconnect the negative battery terminal and leave it disconnected for 10 minutes. After 10 min connect it again and youre done. This will restart the ECU and solve your issue.
There is a fuse which can be pulled which disables the adaptive tune feature for the transmission... So the car always stays on high performance and does NOT adapt to lower performance to save fuel. You can find the thread about this fuse in the v6 section of the forum.
I was just going to try and say what Terminator just said... try this first before anything!