TD4 No power above 2000 RPM when full

[dlibby]

I have a 2003 TD4 which suffers from the following problem.
If I fill the tank up the engine will not pull above 2000 RPM.
As the tank drops the engine pulls above 2000 RPM.
Once I get to about half a tank full the engine will pull fine to over 3000 RPM

Not bothered trying to pull over 3500 RPM

I have spoken to a number of mechanics but no one has a clue including Land Rover ?

Help !!!!!!

[Andrex]

Blocked/restricted vent on fuel filler cap?

[dlibby]

Thanks for that, will check, would make sense .:)