Had this same problem several years back on the '87. Replaced the head gasket and new bolts and the same white smoke out the tail pipe with erratic temps and cylinder pressure in the coolant system. Pulled the head again and found a slight crack (very slight) at the #5 exhaust valve.

Purchased a new #22 head , had the valves renewed and good as gold now with 335,000 miles.

John

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Western NC
1987 300SDL W126 Turbo Diesel 335,000 + miles
1976 300D W115 275,000 miles+