crashmax, there are 3 things that control Turbo throttling. Core voltage, power/current going through the core and temperature. Any one of these 3 items can trigger Turbo throttling where your +1 Turbo will start to cycle on and off rapidly. The harder you push it, your multiplier will then drop down to 20.0 and sit there. Good-bye nice overclock. As soon as the load or temperature drops, your multiplier will return to 21.0.

You need to contact Asus to come out with a fix for this if it is important to you. So far they have a special bios version available for the P6T Deluxe v1 and might do a few more if there's a demand for it.

The Asus rep on XS is Juan Jose (JJ) and has made the v1 guys very happy.
illuminatiASUS

Here are some turbo testing tools to check your own board when running at full load.

http://www.fileden.com/files/2008/3/3/1794507/Turbo.zip