I'm with Leeann -- sounds like something is preventing full coolant flow at speed.
The reverse of your situation is typical, with overheating occurring at low RPMs and going back to normal as RPMs increase.
Over the years, I've seen two cases where the thermostat was installed upside down. The "wax" capsule, usually copper colored, needs to go down, into the coolant.
Your fan belt may be slipping at higher RPMs.
Some high performance water pumps have more vanes ... if I recall correctly, eight vanes versus standard six.
If your fan has a clutch, the clutch may be malfunctioning.
Is your lower radiator hose in good shape? Suction at higher RPMs may be causing an old/soft hose to collapse, restricting coolant flow.
Conversely, a partially blocked upper hose could allow enough flow at low RPMs but not enough at higher RPMs.
Make sure the radiator cap fits properly and has the right pressure rating. A new cap costs less than $10 and is cheap insurance. (I often replace them just to be sure.)
Does your radiator steadily lose coolant? A radiator without an overflow recover tank will have some air space at the top, with the air acting as a pressure cushion. However, the core should remain completely immersed.
(The moose eliminated the overflow recovery tank from our van so the van loses some coolant over time. We just have to be careful to check the coolant regularly to make sure we haven't lost too much.)
Finally, your engine temp sensor or gauge may be giving incorrect readings.
If all these things check out okay, then your radiator is partially plugged and needs to be taken to a radiator shop for cleaning or re-core. (Radiator shops are the only places that can really do an acid flush because the radiator needs to be out of the vehicle.)
