Need more detail: How long on the highway before it starts to overheat?
Happens every time? When the needle starts movng up, does it go back down
if you turn your interior heater on full blast?

Four possibilites right now:
1) Bad thermostat
2) Bad rad cap
3) Blocked rad
4) Fins on rad corroded and falling off.

First of all, don't drive the car until this is fixed. You don't want
the head to warp or crack.

If the engine overheats at highway speeds, the most likely cause is a
failed water pump. Regardless, replace the thermostat first, as it is
much cheaper and easier to do.