Lack of power

Fist thing to check is vaccum,if you don't have a gauge then start it up,rev it then switch off,now pump the brake pedal hard a few times then keep it pressed to the floor while you restart it,the pedal should sink if there is vaccum assistance.
next is all the pipes from the vaccum supply to the servo solenoids and then down to the turbo.
The default position for the turbo wastgate is open,it needs vaccum to close it.
so if there is no vacuum or breaks in the pipe or a faulty solenoid then your turbos effectivle off al the time.
Oh and check the throttle cable if fitted,some of these had a fairly loose arangement and you don't get full throttle from the pedal.

also check when you last had the fuel filter change had a similar problem and it was fuel starvation so i changed the fuel filter and added an injector cleaner all fine afterwards and its a cheap check

As per "1/4 Drive's" suggestion I checked the Throttle Cable in the 406 2.0HDI 90BHP I bought recently and found 10 > 15mm of slack in the Cable. I moved the metal clip forward by 5 grooves to take up the slack and the pick-up in the Car has improved considerably. I also checked the Vacum and that is present. I intend to do the Fuel Filter shortly as well.