You don't say how many miles are on your car but if it is getting up there the crank position (or reference) sensor should be replaced if it hasn't been already. These should really be a maintenance item because they do wear out and can leave you stranded. It's a matter of when not if. An easy DIY, do a search. A bad CPS could cause what you are describing and it doesn't throw any codes.
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1W30TVH0Y1YL0P47J1&year=1999&make=MB&model=E-320-005&category=All&part=Reference+Sensor&appEngines=_any
The "Fits" numbers refer to the last few digits of your VIN.
Good Luck, Dan