What's the oil pressure while this is happening? You might have an air leak on the inlet side of the pump that's causing it to lose its prime. Don't let it knock for very long once it begins, as those babbit rods won't stand much pounding, and the crankshaft can be damaged in a hurry. Also, a clogged inlet screen will drop oil pressure, but if it's newly-built, that shouldn't be a problem. How much work has been done to it? New/reconditiond rods, reground crankshaft, etc., or just an in-the-frame freshen-up? Since it will pressurize by running the pump faster than idle speed, I'm betting on an air leak on the inlet side.
Jerry

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