Does the fuel in your pre filter turn to foam? I had the same problem last year, my pre filters were leaking air into the fuel at the seam. I replaced three of them before I got a good one.
These filters I got from Fastlane and seem to be of higher quality then the first batch:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1AC0K3S9E1JL1D3ZBT&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1987&product=E1000-97870&application=000046825
Splice in a clear peace of PVC hose to help you track down the leak.