I had the same issue with my e39, and now I'm having it with my e46; it started leaking about a year or so after I put on a new gasket. We pulled it apart and determined that the valve cover itself was warped... (no surprise, it's plastic!) Unfortunately the only solution is to buy a new valve cover (and gasket), which runs about ~$250 for parts if I recall correctly. When you get the new valve cover gasket be sure to purchase the 15 or so "o" rings that go around the bolts securing the valve cover to the top of the motor, they're sold separately.
Also, I was looking at some of the original posts in this thread... I have a few good tricks on how to diagnose a failing CCV. Since it completely disrupts the whole vacuum system you can create a few small vacuum leaks to see if there's a blockage by listening carefully to the idle pitch while removing the oil filler cap, or dipstick and then replacing them. Listen to the idle pitch particularly carefully while putting the cap/dipstick back on for any disturbance in the idle. If there is no change then there's likely an issue with the CCV or possibly a vacuum leak elsewhere. A normal motor should sort of hiccup when the Vacuum is restored. Also if the red tip of your dipstick is completely black, the car is using oil and the CCV is likely shot.
2000 528i M-Sport 5speed