I'm not sure myself, but I'm having overheating problems on my W124 too, and they seem to be related to the A/C...in other words, temps are normal when A/C not running, when A/C is on, and (I suspect) aux fans not doing their job, temps start climbing.
I can actually get my aux fans to fire up simply by disconnecting the two-prong lead from the thermostat housing. Did that last night as a test, and let them run for the entire trip home. Despite the 103 temps outside and A/C at max, engine temp remained between 85-87 all the way home, highway-city combo driving! So my guess is that they are not coming on early enough, or not at all!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car