Over heating 300SD, need help

[JohnPowell3116]

Ok so I have had this car for about a year and a half, I'm trying to work with it and I love it to death but i'm really just learning. So to the problem at hand, when I start the car it runs fine, except it takes a little longer to shift then it used to, there aren't any irregular noises coming from it but after about 5 minutes it slowly creeps from its usual 86 degrees to more then 120. So I can only drive it for about 12 miles maybe more before it over heats and shuts off, then I have to wait for it to cool off before I can drive it again. Now the weird thing is it doesn't heat up at all when its idle. I flushed the radiator and check the fans but im not really sure what else to do, can anyone help me?