Don't go in for the most expensive fix just yet. IO had the same problem with my 08 Sky Redline. if you have recently had the coolant flushed, changed or just opened the top on your reserve tank you may have introduced a bubble of air into the coolant that is causing a blockage in the thermostat. The easiest fix is to bleed the coolant and radiator again, then remove the reserve tank from its brackets and raise it above the heater core at least 15 inches. open the check valve for the engine coolant where it enters the engine underneath the "eco-tec" shielding on top of the car. to open the valve, just use a pair of needle nose pliers to loosen the tube attached to the nipple. Then with the reserve tank raised, pour coolant into the tank until it begins to flow from the nipple near the engine housing. when it does, halt filling the reserve tank, and replace the hose onto the nipple. then continue to fill the reserve tank to the cold fill line. Reattach the reserve tank into its mounting brackets, then fill the radiator with distilled water. Run the engine for 5-10 minutes and observe the temp reading. When it starts to level out, 190-205 take the car for a quick drive to insure that the temperature is stable. If it goes up again, you may need to repeat the process. If it takes more than three times to remove any residual air, you probably have a leak in the line that is drawing in air. Always trouble shoot the problem before paying someone 400 dollars for a simple repair.
Sounds like you might have a thermstat sticking sometimes. Would really need to be tested for sure; could have a more significant problem. In the meantime, try not to drive it, or your overheating could quickly cause significant damage...if it hasn't already.
I've recently had the same problem with my 2008 Saturn Sky Redline. Took it to two different mechanics. Checked thermostat and check water pump. Turns out there is a technical service bulletin out for the two check valves for its cooling system. The two valves are there to remove air bubbles from interrupting the flow of coolant in the engine. When they fail, the car will run hot while driving, but will decrease in temperature when stopped. Also the heater will blow only cold air. These are both telltale signs to replace the check valves. As I understand it they are cheap and easy to fix. Hope this helps!
I took my car in for this same problem recently and was told there was a recall for hose. The hose was on back order for over 4 weeks but once it was replaced I haven't had that issue but it appears to have caused a new one. The coolant has leaked out to the point that the low coolant light came on twice since the repair which was about 8 months ago.