Thermostat, air pockets, or lose drive belt. Works fine with the AC on because that turns on the auxiliary fan that I will boldly say you do (or at least should) have.
You are right. You don't have a fan clutch problem. M44 e36s and a manual transmission have an electric cooling fan instead of the belt-driven viscous fan. All e36s have a two-speed electric auxiliary cooling fan mounted behind the front grille and in front of the radiator. If you don't have an auxiliary fan, somebody took it out.
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