running warm at high speeds

hello, i know this isnt really a problem, yet, but im sure it will turn into one, when im drving on the highway (or in town i guess, lol) but whenever i get above 85-90mph, the temp guage goes really high, it doesnt go into the red, so ive never slowed down or stopped becuase of this, but it gets really close to overheating, and right when i slow down to anywhere below 85ish, the temp guage goes back to the middle, almost instantly, you can watch it drop back down it does it so fast, the car has 115000 miles on it, i bought it with 80,000 and have done absoultetly no maintenence or anything to the cooling system,
any opinions?
could this just be the coolant needing changed??

just a guess, but maybe its the water pump..

i recently replaced my water pump on my 2.4l 2004 z24, it was running hot at highway speeds.. not almost overheating but a few lines over normal. after pump replacement it stays at the half way mark all the time.

try to find a lower rated thermostat ( stock is 190) and yes, change yer darn coolant.