overheating prob, kinda....not the maro'

[nikon]

K its for moms Sl 500 ....its a 91' with 50k on it.....we just put in a new thermo and a new fan clutch....still a prob, now only once did we get the coolant to actually boil, and we stuck a pyrometer to her to see if the gauge was lying, unfortunatly it wasnt...Ive ruled out the water pump for if it was bad the temp would shoot up to red in a matter of minutes, it doesnt...only slowly rises, driving will keep the temp from going over the edge, but wont cool it off enough to bring it down to normal operating temp, Im thinking a possible clog in the rad, but other than that I have no Idea what is happening.....throw some ideas at me for any of you that know anything about this damn benz