P0742 OBD2 Code in 2001 Pontiac Grand Prix GT (3.8L V6) - Transmission Shift Issues & Repair Guidance

I recently purchased a 2001 Pontiac Grand Prix GT with a 3.8L V6 (3800 Series II engine). Everything was running fine until today, when I noticed unusual transmission behavior. The shifts started feeling off—subtle at first—but quickly worsened. It now shifts abruptly into second gear, causing the vehicle to run unevenly and triggering the check engine light. My OBD2 scanner has detected a P0742 fault code. After resetting the code and turning off the car, the issue did not recur immediately. However, I'm concerned about what this means. Is the P0742 code indicative of a serious transmission problem? Could it lead to costly repairs like a full transmission replacement? I'd prefer to avoid such an expensive fix if possible. Can anyone confirm what the P0742 code signifies in this model and suggest diagnostic steps or potential fixes? Any insights would be greatly appreciated—thank you for your help!