Are you the original owner?
How many miles?
I asked this question because a problem like this may be covered under the GM Warranty. Sometimes the dealer can offer advice by running your VIN number. They may have a re-call or special fix for the problem at hand.
You need to check the basics, coolant level and battery connections. Think back if this problem is related to any work performed lately, body work or repairs. I mention the coolant because the coolant sensor is triggered off by the condition of the coolant.
The water temperature sensor communicates information to and from the computer. If this has failed to work properly, it would definitely create issues like the one you described. Before we condemn the sensor let's make sure the coolant level is correct and the antifreeze is properly mixed. The antifreeze should be 50-50 one part antifreeze and the other part water. If the mixture is wrong, it could affect the sensor. Most people do not pay enough attention to this little device, which can give you many problems. Learn more about Coolant Sensors, Click on the link provided below...
The first step in troubleshooting any electrical system is to have a fully charged battery. The battery must pass the load test, whatever the amp hour rating of the battery is it is put under three times that amount for 15 seconds and shouldn't fall below 9.6 volts. Example, 65 amp hour battery X 195 amps for 15 seconds must equal above 9.6 volts. Typical starter draw on your vehicle is between 175-210 amps, this must also be checked to eliminate excessive starter draw as a potential problem. Before looking into other areas these test must be performed first. Once checked, it would be advised to perform a charging system check just to make sure the battery is being fully charged.
The key probably has a "chip" in it. A security feature found in most cars today.