Similar problem as well - My car ran in the red with any sort of freeway driving, and under excessive engine load. Replaced the thermostat, and back flushed the radiator several times, but to no avail. Eventually ended up replacing the water pump, and the radiator, and when I pulled the old radiator, it was clear that it needed to be replaced (I could have recored it as well). Now, it runs at 3:00 in all driving conditions.
FWIW - I drove it for about a year with the cooling issue, with a couple trips in the red zone, and it didn't seem to harm anything.