I'm a bit suspicious of the engine fan clutch (just replaced?) based on the vehicle speeds that the problem occurs. Auxilliary fan problems typically show up when engine is at load thermally (airco., high ambient temp., prolonged stop and go, etc.). Temperature rise, at this time of year, when just stopped from speed would indicate air flow problem; ie. #1.engine fan/clutch #2. aux. fan(Gilly has given you the techniques to check that) #3 clogged fins. Just out of curiousity have them check their engine fan clutch installation. Maybe, maybe not.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)