I have a 96 Nissan Serena Van, Automatic, Diesel and the engine overheat?

Mechanics keep replacing hoses, resurfaced and replace cyl. head gasket, checked timing belt, check radiator and yet couldn't find the problem why the engine overheat. Engine works and drive normal except it stop when overheat. Mechanic said it can be done in 3 days but it is more than a week now and no progress.