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I would suspect a TAC (Throttle actuator control) problem. GM Had a lot of problems with this generation of electronic throttle body. GM has recently came out with a replacement cover for the throttle body in lieu of replacing entire throttle body. Sometimes the harness needs to be replaced as well. My recommendation would be replace the cover and the harness. make sure harness is soldered in.
Did you figure anything out?
The fact that it is going into limp mode is suspect enough. It is very likely it has a code. It just cant be read with a code reader and since it is not emissions related would not cause the service engine soon light to come on.