A 98 E320 running at 84C all the time seems too cool to me. The newer cars tend to float around 100C in traffic with the AC going. Check out JimF's website at http://pages.prodigy.net/jforgione/MB_S500.html
Could be that your fan failed in a manner similar to the bimetal strip mod Jim proposes.
My concern would be that the fan is not disengaging as it should at higher engine revs. It might not be balanced at higher revs. I'm no engineer... well I do have an engineering degree
... but I suspect that fans that depart from the pulley and make a meal of the radiator are fans in which the clutch locked up.