Overheating 200 Mercury Cougar

A COUPLE OF DAYS AGO MY VEHICLE STARTED TO OVERHEAT, SINCE THEN I REPLACED THE THERMOSTAT, WATER PUMP(WHICH WAS VERY BAD SHAPE), MY VEHICLE KEPT OVERHEATING AT NORMAL IDLE THE VEHICLE STAY IN NORMAL TEMP RANG, BUT AS SOON i DRIVE IT A LITTLE HARD IT START TO GO UP IN TEMP i THEN SWITCH THE TEMP SWITCH AND FLUSH THE SYSTEM. I DON'T KNOW WHAT ELSE IT COULD BE UNLESS IS THE RADITOR

I would check the radiator fan. Is it running properly. Is the fan for the air conditioning coming on when the AC is running. Check both the upper and lower radiator hose are both of them hot indicating good circulation.
If stuck in traffic turn on the heater to prevent damage to the motor. Do the hoses have a spring in them to prevent collapse. Their is a good chance the radiator is plugged.