96 E300D, the motor runs good sometimes and other times it will not rev up over about 3000RPm with a load on the engine. It also sounds like there are marbles in the motor. The sound is not in the botom end but I cant be sure if it is coming from the injection pump or maybe even the injectors. It acts like it is not getting enough fuel although I do not see air bubbles in the lines. The injectors were replaced about 50K miles ago by the previoius owner

So far I have changed the fuel lines around the injection pump, eliminated the pre-filter going ito the fuel filter, and changed the fuel filter. I also chnged the fuel supply lines from the cowl to the pump. I thought I had it narrowed down to the fuel pick up O ring in the tank but it still does it when the tank is completely full. That O ring is partially down the tube so it should not suck air when the tank is fuiiled.

I have an intermitting check engine light from the vacuum transducer near the EGR valve but it does not seem to be related as it will act up with the light on or off.

It seems to be more consistant in running poorly now.

Any thoughts?