1) Crappy GM tranny
2) Overheating the oil/Torque Converter - highly recommend a supplemental tranny cooler
3) The shift solenoids go bad, there are 2, high and low (1/2 gears, 3/4 gears)
4) Get regular flushes, and change filter once a year.
Maintenance isnt hard to do, and the filter can be done yourself in the garage.
Above all, these are the worst trannies GM ever made.