A prayer to the engine gods, High Rev- High temp- whats the problem? Options

[GT_silver]

So since my move to the mainland i have been doing a bit more fast driving (hawaii isnt really good for that). Hard to tell when the problem really surfaced though. When driving down over 70mph for 15+ minutes in the florida heat my engine will start to run hot. the needle sneaks up to the red, i put on the heater and the needle goes back to its home. At first i thought this was a cooling problem but the engine is running at 4k's around 80... which seems kinda high to me.

It only operates like this in the high speeds and if the air outside is 85+.
I replaced the radiator about 18 months ago along with the pump and most of the hoses.
The fan is kicking on.
The thermostat has been replaced.
And no-one here including our mechanic... can figure it out.

Sorry so long but I'm hoping one of you guys may have run up on a problem like this and can help me find out what the hell is wrong with my baby .

thanx in advance for all your help

[netrata]

You can always try investing in a Koyo aluminum radiator. Wouldn't hurt to keep the cooling system cooler then it already is, especially for long distance, high rpm driving. Like everyone is saying, check for air, loose clamps, and also the coolant overflow reservoir. How is it during stop and go traffic?

This post has been edited by netrata: Sep 28, 2007 - 8:31 PM