This is happening to my V8 SL500 but thought I'd have more responses here and a buddy of mine is having a similar issue with his E430.
In stop and go traffic, both our cars will start to overheat exceeding the 100' point.
Both our cars have had the following changed:
1. Thermostat
2. Fan clutch
3. Coolant
4. Coolant cap
And both of our electric fans do come on.
Any ideas?
Thanks!

 

LOL! No, we haven't "pimped" our cars.
We're old men!
I haven't flushed system out but correct coolant has been used and less than a few years old.

 

I had the same problem in a 1999 Jeep Grand Cherokee that a I used to own and it turned out to be a bad electric fan. The fan would come on and it appeared to work ok but it turned out that the fan speed was degraded. You could barely feel the air moving and it wasn't effective moving the heat from the radiator. Driving at normal speed, the wind blowing through the radiator moved the heat out but in traffic I would overheat. When I replaced the fan the difference was obvious. You can hear and feel the new fan was way more powerful. Had the car 4 more years and it never ran hot.

Are you actually overheating or just moving slightly over 100?

 

The E series is overheating with coolant overflowing sometimes. I've been fortunate so far that my SL has only gone a bit over 100' but traffic got moving again.
Thanks for the advice. I'll check the fans.

 

Just over 100 is not overheating. Fans may only come one at - 110. Check with diagnostics the fan actuation and temps