seriously, its normal for the car to operate @100. MB spec is that it should operate over 80 degrees and the aux fans DO NOT come in to cool the car until it reaches 100. Even at 100, the aux fan is operating at LOW and stops when the car cools to 95.

I think its higher like 106 and than 11? the fan will spin at higher speed and when its a real concern.

from the sound of it, your car is normal with city traffic at 100 and highway at slightly over 80.

It seems its the cap not holding the pressure, remeber boiling point changes as pressure changes, or your radidatoy is really stuffed... While you have the system flushed, change the T-stat anyway... its cheap and might as well do it item.

the fan clutch, just hand push your fan, with engine off of course, and see how many cycle it turns before it stops. if it swing freely, u are most definitely requiring it to be changed. if its not many cycle, like a 1/4 or 1/2 turn, than you are fine. obviously this depends on how much force you push it with.
BTW, you know which is the clutch fan and which is the aux. fan. Right?