I have an 06 Malibu with A 4T45E AND 45k that has had the same concern twice now. The first time it was still under warranty and the dealer said it was the valve body and fixed the concern, that was at 25k, it is now doing the same thing again. when your cruising in town at 35 mph and let off the throttle the converter unlocks and the car free wheels at a 1000rpms and then when you go to accelerate the rpm jumps to 2000rpm and then down to 1500rpm and then a second latter the convertor locks down to 1200 rpms, and it will do the same thing every time you let off and accelerate, I have been a mechanic for 35years and worked on lots of trans but this is got me, there are several valve body valve kit upgrades but none that address my concern it seems like it is the pressure control valve concern but could also be the tcc valve concern. there are no codes in the PCM. Any suggestions would be appreciated. thanks