After roaming though the various pages, I noticed that there were some questions about the "check trans light" flashing on the 4L30E trans. The trans goes into "limp" mode, no power...
Many times you will find that the fluid is scorched or oxidized. Half a can of Berryman's Chemtool B-12 and drive it for a few miles usually stops the problem caused by sticky valves or solenoids. Change the fluid and you are good to go.