Revs staying at 2,000 even when flooring it.

My Mercedes 300SD stays at, roughly, 2,000 rpm's. I find myself flooring the car just to see it climb 5 miles for every 4 seconds. I've changed the first fuel filter and the fuel lines. I have also changed the seals connecting the tubes to the turbo charger. I have also cleaned out the turbo air filter. I have also changed the main air filter on the top. The power loss happened before all of the maintenance and it still continues after all of the maintenance. Once you past 45 mph, it starts to pick up. When I first got the car it did not do this, it was running much more quickly. I have no idea whats wrong with my car, can anyone help me out?