1987 Pontiac Firebird Only overheats when speeding?

Sounds like a bad water pump, stuck thermostat or air/debris in the system.

First, while the engine is cold, pop the radiator cap or surge tank cap. Bring your engine to normal operating temp. (When your fans come on, it's at temp) Then feel the upper and lower radiator hoses. If both are warm, then you know the thermostat is opening. If one is hot, the other not, then the thermostat is stuck closed.

If the thermostat is opening, and before your engine gets too hot, check for the water pump operation/clogged radiator or heater core by reving your engine and watching the coolant for a rise in the radiator or surge tank. (That's easy to verify)

If you don't get a rise, then either the water pump is bad or you have something clogging your system. You'll have to perform a coolant flush and/or replace the pump.