These TSB's might be of some interest.
Component : POWER TRAIN:AUTOMATIC TRANSMISSION:CONTROL MODULE (TCM, PCM)
6T70/6T75 TRANSMISSION - CHECK ENGINE LIGHT ON, POOR VEHICLE PERFORMANCE, DTCS P0716, P0717, P0722, P0723, P0751, P0756, P0776, P0796, P2714 OR P2723 SET (DIAGNOSIS AND REPLACE SPEED SENSOR. *NJ
The dealer should be very well aware of these problems. Exspecially the INTERMITTENT STALL WHEN COMING TO A STOP OR ENGINE CHUGGLES PRIOR TO A STOP, LOW OIL PRESSURE - INSPECT OIL FILTER FOR DAMAGE, NO CRANK, MACHINING DEBRIS ON THE OIL FILTER ELEMENT, NO POWER, ENGINE SPEED REVS LIMITED, POOR VEHICLE PERFORMANCE.
Is that vehicle still under warantee? I believe lemon laws still protect you under warantee even if you are not the original owner.
You may want to file a complaint about that Dealer not having time to fix it.
File A Safety Complaint