so I've had this issue for the longest time now. I replaced the radiator with a mishimoto one. and bought all new hoses and bought the z1 bleeder port too. I also replaced the water pump and a new mishimoto racing thermostat, at this point I think it's probably a head gasket leak that pushes the coolant back into the res where it bubbles and then spills out of the coolant overflow line. I also bought a heater core, but it doesn't seem to be the issue, so I haven't replaced it yet. the only other thing I can think of is the fans, but at over 75 mph I don't think they would really matter then. sadly the overheating caused my condenser core to break and I had just bought it with the mishimoto racing radiator. now i have to replace that too, and i don't want to do it and it break again due to the extensive heat. I did buy mishimoto racing fan shroud which I haven't put on yet due to needing their fan controller, but my car doesn't seem to heat up when just cruising around town, its more when i really push down on it and when i try traveling with it. i have a 2007 G35 sedan sport with the vqhr35

 

Throwing parts at a problem is never a good solution.
Perhaps one or more of the parts that you've already replaced are defective?
Air in cooling system?
Correct coolant?
Water pump?
Thermostat?
Fans?
Defective radiator cap?
Insufficient air flow thru your radiator when traveling over 75?
Incorrect radiator?

Start with the thermostat, radiator cap, fans, possible air in cooling system.
If that doesn't clear the problem perhaps you have a defective water pump?
It is well known that these vehicles do not play nice with non OEM parts.
Good luck

 

Sounds like the coolant system is not holding pressure, rent a pressure tester and make sure it holds pressure listed in the FSM under section CO - Cooling. I think it needs to hold a static 19psi??? Double check with the manual or just use the rating on your radiator cap and pump up the tester to the cap limit.

 

When the system is under pressure it raises the boiling point of the coolant, if it isn't building pressure the boiling point is too low and the coolant boils/expands and barfs out the overflow tube. At idle or puttering around town the coolant temp is probably low enough to not boil, when it's cooking down the highway and everything is thoroughly heat soaked it's generating more heat.