P0742 & P1811 Fault Codes in 1999 Buick Regal V6 3.8L: Causes and Fixes

Hello, I recently encountered a recurring transmission issue with my 1999 Buick Regal V6 3.8L after a transmission rebuild two years ago. The car initially ran smoothly but began jerking during gear shifts, followed by the check engine light (CEL) coming on. I became hesitant to drive it further and parked it for about eight months without use. After restarting, the vehicle appeared fine for several months before the jerking returned—especially in warmer weather and after multiple short stops with the engine turned off between them. The issue seems random: sometimes it occurs after weeks of normal driving, then resolves on its own. Notably, when I let the car sit and cool down, the problem would temporarily disappear. I visited an auto parts store to retrieve diagnostic codes before the CEL triggered again, and the results showed P0742 and P1811. The vehicle runs normally most of the time but suddenly behaves erratically under certain conditions. What do you think is causing this? I’m concerned about potential hidden issues after a transmission rebuild and don’t want to end up with unexpected repair costs at a shop. Any insights or recommendations would be greatly appreciated.