Problem With My Mercedes Sprinter 311 Cdi 2004

[welshblood]

have a problem just cant get any answers too and i mean the right answers, starts runs great, but sometimes runs sound can get 90mph plus out of it not saying you should, but...sometimes wont go up past 40mph and can drop while driving at 90mp down to 40mph, knocking engine off and restating while driving sometimes makes it pick back up, but only after you have passed 60mph its intermitant, so does not show on diagnostic test, and seems to happen at certain levels of fuel, its doing my head in, plus bit dangerous when overtaking to find no power, please help

[TWR]

Check the filters! are they clean & dirt free? would there be a fuel line blockage? Some thing within the tank sucking up to the fuel feed? Are all the inlet/ fuel filter's in good order?