After I got my car back from the body shop it would overheat. It mainly did it while sitting still, but it was like yours it climbed really fast. I took it in to my mechanic, but they couldn't find any problems. Once the learned that the entire cooling system had just been replaced a light bulb went off. The head mechanic said that an air pocket in the system can cause this problem. He said it would work its way out after a week or so. Sure eneough after about 3 weeks total of the car over heating it stoped. I have never had a problem since. Now, I have no clue how the air could get into a system that hasnt been worked on, but this is just an idea.