Are you certain the radiator and/or block aren't plugged with gunk? Another thing that comes to mind is water pump. As for power, is the ALDA adjusted all the way out? Also, there's a check valve in the return circuit on the pump that controls lift pump pressure and if it wears out it can result in power loss. Those are about the only things I can think of, except for maybe filters and make sure there's no fuel restrictions

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint