When you take the vehicle in, the first item they'll have to complete, is a scan of the Service Engine Lamp, for diagnostic codes in the engine's computer. The code stored will dictate what direction their diagnostics go, and may lead to one of some common failures on this vehicle. The fact that the symptoms are intermittent, makes it more difficult to diagnose. Some common malfunctions on this vehicle are the Engine Ignition Module (controls the ignition coils to fire the cylinders with spark), the CKP (crankshaft position sensor, tells the engine's computer when the #1 cylinder has reached it's firing position, this signal also lets the computer calculate firing for all other cylinders), the fuel pump (pushes the fuel from the gas tank to the engine). At your current milage there arn't too many other items that fail, and are affected by temperature. The first thing I would do is have the battery cables checked for being clean and tight. Often, earnlier GM's the cables become slightly loose, causing a voltage drop to the engine's computer and thus affecting the performance. If the cables are corroded, have them serviced as well. If you take it to the repair facility and they make a diagnosis, your welcome to post the results so that we could go over it together. Also, make sure you choose a quality repair facility, with a good reputation. Often faults like this are difficult to diagnose and you want an educated technician working on your vehicle, not learning on it at your expense.

-Rip