Check/Replace the Accelerator Pedal Sensor. Check the harness for any wires pulling out of the plug. Also, there are two grounds that can come loose on the back of the block below each cylinder head. I believe they are ground G103/G104. Lastly there have been issues with wires and pins coming loose in the large connector that plugs into the throttle body. I believe the brown and yellow in particular. The part number is PT1572 or GM# 88986255. It is about $100. If that is the case you need to cut and resolder one wire at a time as all the wires are white on the replacement harness. I have heard that GM is not being very cooperative with people when addressing this issue. Hope this helps.