Why do I have coolant blowing out of the overflow tank?

I have a 2003 Dodge Dakota V6, 2wd. After 2 to 2.5 hourh of constant 70+ mph interstate driving coolant starts discharging from the overflow tank outlet causing the vehicle to overheat. I have replaced the radiator, cap & thermostat. I have had the system pressure tested and no other leaks appear. There is no evidence of oil in the coolant or vice versa. What else could be wrong