I have a 2000 Ford Expedition 4.6L engine. Truck turned off while driving. Tech replaced driver side cylinder head and had passenger side Cylinder head machined to acceptable specs. Truck is running smooth but one thing. I have been experiencing sudden oil pressure drops. Tech replaced oil pump and still same condition. Tech replaced Half Block which he told me was the lower half of engine due to leakage, STILL same problem!Time varies in oil pressure drop, it'll last from 2-3 minutes to 30 minutes after the engine is started till condition occurs.