A few years ago I had just read an article in a magazine about " having enough vents, breathers, and PCV valve " in your valve covers to rid crankcase pressure/vacuum...

Not 2 weeks after this my mechanic friend had a NEWLY rebuilt 454 chevy, that had just been installed in a PU screeching just LIKE YOU , show up at his shop.. My friend is a GREAT mechanic but he was stumped on this one, just getting ready to pull the pan when I showed up.. I am by no means a great diagnostic person, but the article I had read hit me and looky looky, a PCV valve in one valve cover with NO VENTING on the other side at all.. Pulled the OIL FILL RUBBER PLUG on the other side and it was fixed...

Sounds like you have the same problem.......................