I had a problem like that once. I put in a rebuilt 351 that I bought from a guy cheap, it had leaks all over. As I corrected them I started having this same problem, what I did was pull out the PCV valve from the top of the valve cover and modify it to allow more flow. Mine had a little weight looking thing that acted as a baffle I think. I just cut it out so it could flow as much as needed, with the pressure I was seeing, didn't think I needed a baffle. worked fine for years. Don't know how the PCV valve is setup on a these cars, but the principle should be the same. I always wondered if the engine maybe had a blockage in it somewhere, or a gasket that wasn't sitting right, but, the car is still around 5 years latter and the guy that owns it now still hasn't had to do anything to the engine.