When you go under the back of the car, check the fuel hoses that connect the fuel tank with the hard fuel lines. Usually these have never been changed, and apparently the 20+ YO braided lines get all weepy when you push BioD through them. Could be air getting in the lines.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT