There has been some talk in the past about poor sealing surfaces on the thermostat. Since this is a bypass system it's possible that is causing your problem. I would think a non-contact thermometer would show this problem since the radiator would be pretty cool but the block pretty hot.
Check the oil cooler too with the thermometer.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

It seems that the problem is the car can't throw the heat off when the ambient temperture is high. so, something can't dissipate the heat. WP is ok and pushes the coolant thru the rad. rad can't get rid of heat. bad rad? Maybe you have to look at more than the fins. rad could be plugged with excess permatex you mentioned earlier? Get that IR tem gauge. great investment. I used mine to check gauges and found one of mb temp gage off by 10 degrees too hot.

__________________
71 220D 169K wrecked
83 240D 118K sweet 4 speed
91 350SDL (one of the 60% good engines) 156K
84 300d (loaner to my sister) 189K
79 300SD (partswagen)
86 420SEL partswagen
70 220d (partswagen)
68 280s GASSER!!! under construction now
85 300sd 310K miles winter beater car retired
93 300d 2.5 turbo 168K wife's car
83 280SL euro 5 speed 155K
69 250S newest project 54K

Had similiar problem in a 455 olds. Problem was small leak(s) between the combustion chamber and a water jacket, brought on by a bad thermostat that caused a slight overheat. Fixed only by head gasket replacement.