Double-check radiator for cleanliness. Include screen pre-filter in assessment.
Try washing radiator repeatedly front-to-back and back-to-front with open-ended water hose (no nozzle, bends fins). May take several times both directions to work loose the accumulated debris and crud.
Avoid pressure washing.
Check oil-cooler too.
If needed use an HVAC fin-comb to straighten and realign fins for maximum heat transfer efficiency.
After cleaning try using a cheap fiberglass home-type 'cat hair catcher' $1 HVAC filter positioned ahead of the mesh pre-filter and/or radiator.
If radiator is truly gunked (oil, fluids, sticky antifreeze) try using liquid dish soap or commercial HVAC condenser cleaner.
If everything else looks good per prior posters, start considering possibility of thermostat problems.