It sounds to me like you have pressurized
oil escaping some where in the system. I ended up switching my 428 block to threaded plugs to ensure that the soft plugs didn't get pushed out of place. I've seen one of the rockerstands break and let
oil pressure drop to a point where the original engine owner let a friend of mine buy the engine for little or nothing, my friend found the broken rockerstand problem replaced the stand and used the engine for years later. I've also heard of the releif valve in the oilpump getting stuck in the releiving position. A leak almost any where in the system could cause your simptoms. Gasket sump to pump. gasket pump mount to block, any of the gallery plugs. Did you do anything with the front center gallery at the front ? Originally a 428 had a soft plug with a bent cotter pin. It can be replaced with a 1/4" pipe plug with a small hole to quickly evacuate the air in system at start up. A small hole will let a lot of air out but less
oil. That is what I currently have in my sideoiler now, but it would be more necesary in a top center oiler like a 428.