The intermittent nature of the problem is highly suggestive of a failure of the check valve in the fitting where the hard line connects to the pump. The parts of the valve rattle around in the pump inlet and cause intermittent blockages. You can remove that part with the pump in place on the engine.

Note the small disk in the right-side photo:
http://catalog.peachparts.com/ShopByVehicle.epc?q=1982-Mercedes--Benz-300sd-Diesel--Injection&yearid=1982%40%401982&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6206%3AMBC%7C1510%3AED%7C10000134%40%40300SD&catid=240706%40%40Diesel+Injection&subcatid=240735@@Vacuum+Pump+Check+Valve&mode=PA
If you remove the nipple and that disk isn't there, you will need to fish the disk, valve and spring out of the pump inlet, then replace the valve with a new one.