All 1999-2004 GM RWD cars and light trucks(including 2003-04 Hummer H2's) with the 4L60E or 4L65E automatic are susceptible to transmission slippage, erratic shifting and/or a loss of 3rd or 4th gear.In addition, the MIL may be illuminated with DTC PO757 stored in computer memory.According to GM, all these problems could be the result of metal chips or debris plugging the bleed orfice of the 2-3 shift solenoid. The code could also set if the 2-3 shift or shuttle valves get stuck in their bores because of the junk. Getting rid of the debris is crucial to a successful resolution.
Start by inspecting the2-3 shift valve, the 2-3 shuttle valve and their respective bores. If need be, clean the valves and bores with suitable cleaner, then blow-dry everything with compressed air and reinstall the valves. Now move on to the 2-3 shift solenoid. Remove the solenoid and clean it up with solvent.if the opening has a screen to prevent clogging, reinstall the solenoid. If it doesn't, replace the solenoid with an updated, screened verison.(part no. 10478131.)

Yeah, see it all the time at work. This is how I make my living. Just drop the valve body and spacer plate down (you don't even have to take the trans out) and clean as mentioned above. It is a small chip of aluminum (either from the valve body or case from machining of the parts at the factory) that eventually finds it way and blocks an orfice bleed in the spacer plate. I have also seen chip debris stuck in the 1-2 accumulator valve resulting in a hard 1-2 shift. This concern is often intermittant and requires only a pan gasket, and both valve body spacer plate gaskets. Keep in mind that in 2000' they updated the spacer plate and requires different gaskets noted as "CA" and "VB" instead of "C" and "V" of old. Mixture of these gaskets and spacer plate on a new style valve body will result in slipping, long shifts, nuetral flares and worn 2-4 band. Good luck to all.