1995 Ford Mustang fluctuating temp

While accelatating the car the temp gauge starts to climb to about 3/4, it also happens when the car its going uphill, so in other words if the engine is under high rpms the car starts to overheat. Once its going at a steady speed it fluctuates from 1/4 to 1/2. What could be the problem, has new radiator the coolant level is good and the temp sender was also checked and replaced.