Check the bolt that connects the canister fuel filter. You may be getting air into the fuel system. I don't remember if its an o-ring or crush washer on that connector. How's the engine run at idle? Does the exhaust have excessive smoke?

Could you be more specific on "lost of power". Does the engine not rev up? Or does it rev up but with no power?

The tank strainer may be clogged. I believe you can swap the intake and return lines to check. Also how full is the tank? When was the last time it was filled?
(Just trying to cover the bases)

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92 300D 2.5L OM602 OBK #59