The GM dealer should be able to test your engine computer to see if it working ok.
I really doubt it's the computer, but if it is:
For a used computer
www.car-parts.com
www.rockauto.com (new)
Check each of those wires for resistance which would cause a voltage drop. Use a ohm-meter and stick a small push pin (or needle) directly into the wire if you can't get an open contact.
I would just bring the car into the GM dealer and have them use the Tech2 to see what is wrong.