Hopefully I can help..
What is the mileage on the vehicle?
What its service or recent repair history like?
Is there any sort of misfiring when this problem occurs or warning lights come since or prior to fault?
Mileage is 102,000
No repairs ever, serviced each january with mot
No warning lights or misfiring
Hi..there thanks for your reply..
Ok..If it was serviced recently there then you can sort of rule out fuel filter/air filter problems here..but check the air filter anyway just to be sure..aswell as that if you can follow the pipe work all the way around the engine checking for any obvious issues or small tears etc.that you may come across..especially so around the inter cooler piping as any faults there can cause black smoke lack of power etc..one other item that may need to be checked out is for faulty or stuck even EGR (exhaust gas recycling unit) very often those can produce very similar issues as you describe..these can be removed and cleaned and then refitted and then see what that does..sometimes that can work depending how badly carbonised up the valve is..if once removed its heavily sooted up then that could be your problem.also check for stuck waste gate at turbo.obviously other things like fuel pressure/injectors problem can do the same but if there is no misfire then the injectors should be ok to test those would require specialised equipment that only a diesel specialist would have..But as above have the intake/intercooler system circuit checked for issues/restrictions aswell as the egr valve checked out and have cleaned out..and see if that helps..