Mechanic Q: Why is my car overheating??

My car only overheats after about 20 min on the highway at about 70mph. Both gauges start to rise and I have to keep my heat blasting to keep it from reaching H. I have to stop and let it cool down after about 45min. The resevoir is full from overflow, and both hoses hot. It doesn't do it driving around town usually. It is a automatic 1998 Dodge Avenger ES, 4 cyl. Help!