No, it's not the fan or clutch. A very common problem is the gauge temp sender goes bad, they start reading lower than actual. My fixxer-upper 1987 300D has that problem right now, it reads 75°C when I know it's more like 85°C. A new sender is cheap, p/n 005-542-26-17, ten bucks for a new one. It's located approximately below the #2 injector, behind the injecton pump - tough to access. You need to drain the radiator before installing it. Now if you've already replaced this sensor... then yeah, try a new t-stat. But I'll almost guarantee that after you replace this sensor, your gauge starts reading 85°C most of the time.