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I just fixed a very similar problem in my car. I can't tell all of your symptoms from your earlier post, specifically, what does your turbo guage read when you are under full accelleration.
Anyways, for my problem, the symptoms were that it was stuttering and hesitating on accelleration, and the turbo guage only showed a little bit of pressure. The performance was crappy. I traced the problem down to a leak in my intercooler hose. Here is a description of the problem: the computer reads how much air is injested by the engine with an air mass sensor hooked up on the top of your air filter. Based on this air flow, it calculates how much fuel gets spit into the cylinders. Now, consider a turbo engine that compresses the air. Then consider a leak in the pressurized system - some of the carefully measured air gets dumped out of the otherwise-closed system. There are two results - your actual turbo pressure is significantly lower (with lower performance), and the engine injects too much gas into the cylendar compared to the air that actually makes it there. This makes it run rich, and be a real dog on accelleration.
For an older engine, the EGR system will throw your oil fumes from the crank-case back into your intake system, resulting in oil accumulating in the rubber pipes. These corrode with time and develop substantial leaks. The intercooler is also susceptible to plugging up (via condensation?) in the winter (for us northern folks).
I highly recommend that you confirm that there are no air leaks in your induction system, paying particular attention to all of the rubber components and the exposed bottom of the intercooler device. I also recommend a smoke-test on your vacuum system to ensure that there are no vacuum leaks anywhere- it does not cost very much.
One additional thing to check - your turbo has a pressure regulator on top of it that has a rubber hose that goes to the intake manifold. This pressure regulator is spring loaded to pull the wastegate shut via a link rod. If the pressure in the intake manifold gets too high, the rubber hose transfers this air pressure to the regulator where it overcomes the spring tention and pushes the rod out, opening up the wastegate, and reducing the turbo pressure. I believe you have checked this hose, but I would also recommend that you check that the link rod is still connected to the wastegate, and that the rod still moves. You should be able to pull the rod manually towards the regulator about 1/8" with your fingers; you will feel spring tension pushing it back. Just make sure it is not seized and it is properly connected. Mine came disconnected and I had it reconnected and a spring washer installed to keep it there.
One final gripe - with the automatic transmission having such a steep gearing ratio that allows for accelleration to over 70 km/h in first gear, this also makes accelleration from a stop sign very sluggish. Couple this with a very tight tourque converter and a turbo engine that doesn't really go until 3000 rpm, .... Lets just say that performance accellerating from a stop sign to city-speeds is less than desirable. Add the above turbo issue, and you might was well get out and push. My '90 740 turbo with a standard transmission smokes the performance of my 850 - until about 70 kph. But that is another story.
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