On a brand-new engine, this definitely shouldn't happen - with an old one, it could have many different reasons. Assuming that everything on the engine is fine, as it came straight from the factory's test-stand:
You are probably still using the old propeller, right?
I'm not a mechanic, so just guessing, but the Nitrogen-pressure may be low. Otherwise, it may mean that there is not enough oil pressure for some reason.
Does the problem get better or worse after a while, once the engine has warmed up?