Discovery td 5 misfires at 4th gear

Hi I have 2000 Td5 XS auto at 137,000 km. When I try to accelerate on the highway while crusing around 100 kmh it starts to misfire at 2500 rpm. I tried to accelerate at lower speeds and lowers rpms at different gears it runs smooth. After testing it several times I concluded that it only misfires at 4 th gear above 2500 rpm Any suggestions? Thanks.

I ll answer my self... after tool box showing short circuit at injectors 1.2.3. and faulty air flow meter it turns out its the stuck turbo valve... repair cost 35 pounds...

wat did you,re actual repair involve i have the same sort of fault but only intermittant. sort of get round it with gear change and revs but would be interested in the fix
cheers

Mastermac, The mechanic changed the Wastegate Actuator, at first this fault did not display at the computer but when we check the truck after the repair, shortcut at the 3 injectors and airflow sensor disappeared...