Apologies if this has been said, and assume my cooling system/rad/pipes and so forth is all ok.
I run just an electric engine cooling fan. My car in stock form the fan stays off until around 65 -70 MPH, @circa 40 degrees C if I try and hold 80 MPH for around 10 -15 minutes though the fans first speed comes on for awhile then goes off then repeats, as long as I stay at 80, go back to the 65-70 and it never comes on unless climbing a long steady hill. Even when it is 30C outside the temps still rise when you start to push a little harder, I do have a modified temperature gauge that actually moves with engine temperature, not the stock offering that sits neatly in the middle of the scale until your near overheating. I can watch the gauge rise if I push a little harder, the fan switches though are a known temp so this is not speculation it is fact.
So remembering that the power to push vehicles goes up by a factor of IIRC 8 then this proves that the stock cooling system needs a little help at higher speeds, even the stock viscous will come in at speeds circa 70 - 80, it seems the 80's shape puts quite a bit of load on the cooling system at higher speeds. So on mine I blocked all the air gaps between the bumper and radiator, in effect preventing any air hitting the front of the car escaping from bumper to rad face for example, this actually stopped the fan coming on until holding at least 90 for n extended time. So check around the radiator for the sponge that fits between the rad and car body, it normally rots away, also the plate between the chassis rails (protects the fan belts from intrusion underneath) seems to have an effect if it is not there. The reason for that I am not sure, but it affected mine but again only at the higher speed.
So, you could have a weak viscous, and/or the blocking sponge is missing?
Next, have you had the exhaust route relocated, I have seen some lift the exhaust up nearer the transmission fro ground clearance, the result is more radiated heat nearer the gearbox.
I am no expert on auto trans and Jon has made a good point but, the cooler receives fluid at idle from the front pump right and during all times the pump is operating? I do not recall seeing a valve or anything in there that would interrupt the cooler flow when the converter locks up, absolutely open to this being challenged as it is a long while since I pulled one apart.
Those are just a couple of thoughts Lorin.
regards
Dave