Maybe you have checked for this...........
but this really sounds like a large air lock in the system.
If so, try this: put the front end up on ramps/jack stands, while cool, and disconnect the upper radiator hose. Then fill both the radiator and upper hose till they can't take any more. Reconnect the hose and run till warm then top off.
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Sam
84 300SD 350K+ miles ( Blue Belle )
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