Likely its the head gasket. I went through the same issues as you on my '84 300D. Ran like a top, good heat, starts perfectly, etc. Used very little coolant to speak of but the coolant was turning dark from soot and the system was pressurized all the time, hot, cold, etc. If you can do an engine swap then a head will be easy. Recommend Metric Motors for a re-manufactured head. Do a search on my posts and you will see what I had to do. Oil analysis indicated coolant in my oil. RT

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When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K