I start to have the same problem with my 2003GC, around 69,000, it started does that. The transmission will not shift down from the 4th to the 3rd around 45MPH. shortly after the packing solenoid started to leak. I took it to a repair shop and the replace the solenoid and changed the fluid. The car continued to do the same. I took then to the dealer to flush the ROM but it didn?t help. It actually got worse and the car started to hard shift from 1st to 2nd. After going back and forth, the dealer scanned the transmission and told me that the front pump needs to be replaced. I pulled out the bulletin from the web were it stating that the front pump in the transmission need to be replaced. The cost of the overhauling the transmission is $2200. I dropped the pan, changed the fluid, must be ATF+4, added transfix, and drove the car several days on 45 mph on cruse control. The problem seems to correcting itself for now but I still have the hard shift from 1st to 2nd if the speed 10-15 mph and the RPM is 1500