Jerky gear shifting and code P0742

When I start from a complete stop the car has no power, slow acceleration and the gear shifts are really hard and jerky. Starting normally from a standstill the car will accelerate slowly and the rpm's will climb up to around 4500 before the transmission will shift into the next gear. When it does it will be very hard and then the car will be jerky, like when driving a stick and you drop the clutch with to little power and the car jerks you forward and backward in your seat. As the speed increases this smooths out until the shift change and then it does the same thing again. Once you are up to speed and driving the car runs fine. The check engine light is on so I hooked up my OBD II scanner and it gave me this code: P0742 - Torque converter clutch stuck on and another one for the EVAP system indicating a leaking filler cap. I have drained the transmission fluid and replaced the filter. The old fluid was not really bad and the magnet showed normal wear accumulation. I checked the wiring, though I am not very knowledgeable about the electrical system, so basically just looked for a broken wire or plug around the transmission (don't even know what harness to look for). The car still does the same thing though. So what could I be looking at? Is there something I could do maybe? Replace the transmission or try and repair it? Is something mechanically wrong with the transmission possibly? Any help or guidance would be greatly appreciated. Thank you for your time on this.