2007 Grand Prix in Limp Mode

It does sound like an electrical issue. Bad grounds, loose or corroded connections, even a weak battery can make modern cars do all sorts of weird things. I'm presuming whoever is working on your car is an expert on automotive electrical systems and fully understands how they work and how to trouble shoot them. (Not all mechanic/technicians do) If not, find one who does. I am fortunate to have such a person who often finds issues with my older vehicles. Its slow going even for him and my repair bill is often 50% diagnostic time and 50% actual repair. If you're a do it yourslfer, have your battery load tested to make sure its 100% strong, Locate all of the several ground points on your car and take them apart, clean everything, inspect the wires, reconnect and tighten them and make sure all wiring it properly tied in place and not flopping around. Scan in regularly for trouble codes, but even then don't just replace parts because they show as not functioning. Sometimes bad wiring or other issues will send a fault reading to the computer. There's lots of online info available that addresses each code and its many possible causes. Good luck. I know how terribly frustrating something like this can be.