Those stock 4-blade radiator fans just don't have enough pitch or blade area to pump much air at all at idle speeds. My 544 used to overheat rather quickly in stop and go traffic (I'd have to turn the heater on full blast, even if it was already 95 deg outside!) or even on a cool day if you just started it from cold and let it idle for long enough. And when the engine is revved up you are most likely moving along, and the air pressure on the front of the car does most of the work. So the stock fan doesn't work when it needs to, and does when it doesn't.

I finally solved it on mine by installing a large (15 inch) thermostatically controlled pusher fan in front of the radiator (scrounged from a pick-n-pull for $15). It very rarely comes on - if the car sits still it will kick on for about 30 or 40 seconds every 5 - 10 minutes. I took the fan blade off the water pump and tossed it on the pile of obsolete parts.

PS - It also almost completely cured the boil-over problem it used to have. After pullling off the highway and shutting the engine off at a gas station it often used to briefly boil over and spit a few cups of coolant out. Now the fan will cycle a few times as it is cooling off and cools the engine down before it can boil the coolant in the block.