If you get a whoosh sound when you remove the fuel cap, you definitely have a clogged fuel tank vent which will cause power issues. Usually it's at the flared plastic fitting under the car. You'll want to snake something like a wire or pipe cleaner up there and get it unclogged. You can also used compressed air, but there is a risk of whatever is clogging the line to go inside the tank.

Also, if you replace the O-rings in the lift pump it should restore the performance and that will rule out another possible problem.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles