On hot days with the a/c on, my water temp will start to rise. I shut the a/c off at 100° to prevent damage, although I'm sure that the cast-iron OM617 can handle it. This is with a non-operational auxiliary fan, and I also believe that my engine fan clutch is dead.
Have you ever heard the engine fan activate? If it's anything like the fan on my truck, the car will all of a sudden start doing its jet engine impression. In addition, the car might feel like it's losing some power, but I'm not sure on that. If you're not getting this, try spinning the fan with the engine not running...if the engine's cold, it should turn but with difficulty.
My guess would be that the engine fan clutch is dead but that the cooling system is otherwise happy. I'm sure it could stand to be flushed out, though...I know that I need to do that with mine.
Hope that helps some...good luck!