Have to agree that there is probably air getting in somewhere, had a similar problem with my 190D a few weeks ago.

My first ride was a 55 Ford pickup (gasser) that developed a similar problem, except that it would quit randomly. After replacing filters, fuel pump, fuel lines, etc, I finally resorted to pulling the gas tank and found a maple leaf drifting around inside. Would wrap around the fuel pickup and stall the truck. Once engine was dead the damn leaf would float away from the fuel line and all was ok with the world until the bloody leaf would drift into place again.