I had this problem with a customer's van and it turned out to be the valve
cover itself.  Apparantly it had gotten bent only slightly (it looked fine)
but when I put a new one on it sealed up fine.  I have these new for $15
each.  You should be able to use one of the valve cover gaskets you already
have since there is nothing wrong with them.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242