Another place to look as a cause would be excessive crankcase pressure. This is caused by blowby past the piston rings which pressurizes the case, then finds its way out through the weakest joint (valve covers?). The engine has a normal vent to the air cleaner. Might this hose be plugged?
If you've exhausted the gasket/flat valve cover route, try a compression/leakdown test and see if everything is okay.
Sherwood Lee
http://members.rennlist.org/911pcars/