Try a pressure test of your cooling system. Marginally failing parts can be very frustrating to find but a pressure test will help stress them a bit. In my case it was the plastic hoses behind the oil filter housing on the side of the motor had distorted O rings. Does your heater work OK?

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2001 x5 3.0 (sold with broken motor), 1990 e30 M3, 1991 318is, 2002 325i, 2008 335i Dinan stage 3 6 speed,