This sounds like a partilly clogged cooling system. Take a look first and make sure the system is full and is not low on coolant from a leak etc. Take the cap off the rad and take a look in there to see if its low. Also, take a look in the rad and see what it looks like. Does it have a lot of build-up/rust deposits? If the coolant hasn't been changed for at least 4 years, it would be wise to do a complete and through flush and refill, especially if your using dexcool.