What could be the problem to the loss of power and always moslty half on engine temperature guage.

I recently travelled 1000km to a destination and within the distance after travelling about 400km the car just lost power and could not pull as if the plugs needed replacement for about 80km, and i stopped for at least 2 hours thinking to change the plugs but when i started it after it had cooled down everything was well again and it drove up to the destination whithout any problems, the temperature always varied between the first quater and half although i was driving on a highway, what could be the problem??