First off check the short plastic line that runs from the intake manifold over to the alda switchover valve, if its plugged that's your problem!
The 603 turbo uses a flap in the exhaust that is activated by the wastegate, and they have been known to get crud that prevents them from closing completely so the result is low boost at take off. The symptom is the car finally seems to get up to normal power (or almost normal) after about 10 seconds or more because the turbo isn't spooling up properly. The remedy is to clean out the stuff around the flapper. You will see what I mean when you have the turbo out. I suggest using some liquid wrench or PBBlaster or Kroil on the studs, the nuts tend to freeze on the threads associated with the exhaust. Its not uncommon to break a stud (lower rear I think)
Another possibility is if you still have the trap oxidizer installed it can make the turbo seem to be inoperative, when in fact the Trap (the Nasty Trap Oxidizer!) is preventing exhaust gasses from passing thru. I certainly hope yours doesn't still have it on the engine.
Lastly, pull the crossover and see if its plugged. If its almost clogged then you need to remove the intake manifold also and clean out the gunk that tends to collect in them. And disable the egr to prevent this from happening.