Check your fluid level and fluid color. Is it dark and smells burnt? If it has an odor of a burnt fluid it is more likely that you will be needing a complete transmission rebuild.
Sometimes this type of problem is associated with bad shift solenoid. Have your shift solenoids check. A good mechanic should have the resistance specification at hand when checking these solenoids. GM have a recommended resistance in ohms of properly working solenoid switch.
Check the transmission's wiring harness. GM cars are known to have a pretty weak wiring harnesses.