Have you had your car in for recall work? I found the same for mine. One of the problems of the YD engine was that the turbos were susceptible to exploding, caused by the ECU sending the wrong boost signal. The recall will replace your ECU with a Denso unit. After mine was replaced I found that 3,000 to 4,000 RPM was much more lively.
Word of warning though. If you plan to remap your ECU then first check your ECU. If it is made by Bosch then that's good. Keep it but get the remap done ASAP. If it's Denso then go get the recall work done. You see Densos are very hard, if not impossible, to remap.
Have you had a service recently? Loss of power could be caused by an already damaged turbo. Service might pick up on that. To be honest mate it could a lot of reasons. Dirty injectors, faulty diesel pump. Get it checked out.