For what it is worth, go see a mechanic.
My thinking would be that if it can handle high outside temperatures whilst sitting in traffic then the cooling system should be good.
The question then is why is the running temperature increasing at increased revs and speed which also increases the circulation of coolant & cooling air passing through the radiator & around the motor???
If it is not in the basics of points/plugs/timing/fuel supply etc. ( tune up stuff) then perhaps a mechanic can give further info or tests.