If the stumble/hiccup goes away when driving in Sport mode or when in cruise control, it's the TC. Sport mode increases the TC lock up solenoid pressure so it doesn't lock/unlock every 2-3 seconds. Cruise control sends an input to the trans ECU to keep the TC locked up as much as possible so the trans isn't shifting constantly with variations in the road - hills, etc. Good simple test. Mine has been doing this for 10k miles now. I just drive in Sport mode and all is fine. However, the inevitable trans rebuild is looming. If you have banging shifts, bad shudders at certain speeds, etc, you probably need to get the trans done sooner than later. Good luck.