question... how do you hear water running through your engine when you accelerate? excuse my ignorance.
(and, while low compared to the 216k miles on my car, i wouldn't call an engine with 100k miles on it "new")

did you notice if the fan was running when you turned the car off?
has the water pump ever been replaced?

sounds like your thermostat may just "sticking" however, there are other problems that may cause these symptoms too.

i would say just check all the fluids to be on the safe side and keep an eye on it. if you havent ever replaced the water pump, go ahead and do that. and the timing belt while youre at it if that hasnt been done.

just my 2 cents...