as Luckyf8 says this is a common mystery that hasn't been solved. happened to me too after head gasket job - thermostat started sticking. replacing with new Behr 87C produced constant 90C with 95-100C+ in heavy stop and go traffic. a couple of things that have been mentioned in the past are replacing the pressure cap which i haven't done yet and checking / replacing the overflow line from the reservoir that goes into the wheel well. if the coolant concentration is greater than 50% it's worth a couple of degrees taking it down to 40% with some water wetter added for good measure (but not significantly so in my experience). i myself have resorted to an 83C thermostat which has brought back the 80-85C temps i was accustomed to. i should have tried the cap first and i may do that anyway...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"