1998 Porsche Boxster overheating problem?
 

I have a 1998 Porsche ATEQ VT55, which up until recently, has been running flawlessly, now it overheats. There are no leaks what so ever when it runs, and at idle, it takes about 30 minutes for the car to heat up near the highest temperature, at which point, I shut it off. There are no leaks what so ever and the fans on both sides are working fine for the radiators. No unusual noises can be heard when the car is running, and everything works fine.

What could be the problem? Also, is there anything I may do to see if the problem is really it for sure? I'm not trying to replace my entire cooling system. I'm hoping it's just a bad thermostat, or maybe even a bad water pump or something, but like I said, no leaks and no unusual noises.