I agree with Mike.  Sounds like limp mode and we need to know why.  It’s unlikely wastegate adjustment unless someone has already been messing with it.     Nigel

"A false friend is more dangerous than an open enemy." Francis Bacon

I agree with limp mode.....it should go away after the sam cycles but it will always come back if your waste gate isn't set up correctly. IMHO, the waste gate needs to be set so the boost pressure never goes much above 16 psi or you risk always having this happen. Do you have a way to see your boost while driving such as a ScanGageII  ???   IF you can afford to get one buy one......they may go on sale for black friday or christmas  Boxing day etc etc. They are excellent for a smart car and it's issues so to speak. You can also watch your fuel consumption rates as you drive in real time to get the best litres per hundred kms driven.  Along with being a scanner.  Ask anyone who has one if they are worth the money I bet 99% will say it is. Once you can watch your boost as you drive then you can adjust correctly the rod locking nuts for the waste gate and thus be able to set it correctly under full acceleration as you need to be able to do.  

Sounds like limp mode. Run a diagnostic on the OBDII.

In the past I found the 05 cdi would go into limp mode if I drove high rpm with a cold engine. Reset and all would go back to normal. Yesterday, highway speed, 6 gear, 100km/hr and the car starts to slow down, give it more gas until I am flat on the floor hovering top end at a 100 but  slight rises and she would slow down to mid 80's (still flat to the floor), 5th bring it back up to 100  (3500rpm) back into 6th.  I am leaning to a turbo issue but at slower speeds I can hear the whine of the turbo.  I have not popped the hood yet but what can I expect to find?