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

[celicamike]

My '94 GT does the same thing, and I would also like to know why. It only overheats at high speeds when the engine is "cruising" at 4,000 rpm for an extended period of time, so personally I think that this is the cause of the problem. So now I just don't go faster than 65 mph. It pisses people off on the freeway, but my engine is cool and my gas mileage has never been better.