Could mean a number of things.  Is the waste gate opening properly?  Is the mixture too rich, or do you have fuel burning close to the probe?  You are probably right figuring the probe is faulty, when all of the other indications are in the green, but should be troubleshot to make sure.  Taking care of the turbo is very important as they are quite expensive, however, they should last a long time if they are maintained properly.  Have your mechanic take a look at it and see what he finds.  Here is a couple of links to help you understand better how a turbo works.  I hope they help:

http://www.ramaircraft.com/Customer-Ser … ooting.htm

http://www.kellyaerospace.com/articles/ … harger.pdf