04 CTS 3.6L Tranny Problems
 

I'll try to be brief but a bit of history first. About 4 months ago the car would sometimes not take off from a stop. Then would lurch forward all of a sudden and run fine. Check engine light would come on. Once went into safe mode while going down the highway. Took to a dealership and they reflashed the program. Didn't work. Check engine light came back on and we went back to the dealership. They couldn't recreate the problem and only codes being thrown were for ignition loss on cylinder 6 so we had spark plugs changed.

Now has hesitation going from stop to first like torque converter not working well. However, no problems at all going from 1st to 2nd and beyond except will now not shift into overdrive.

I'm tempted to throw in a used tranny just to have it running right so I can sell it. However, if it's not an physical issue with the transmission then I'll just be wasting my money. Does anyone have any ideas on what could be causing this or what to check? Car has a little over 100k miles and no prior transmission service. Thanks!