AutoCAD viewport changes

[Thomas Bolt]

I have some delicate Rendering to make, where Camera and Target are to be found in nature. I can manage to find the correct perspective with lots of helping points in the original picture or oblique photo. So in AutoCAD I make the viewports (VP), all with DVIEW and the respective options for zoom, distance, pan, camera, target etc (mainly by try&fail).
Once I have the VP adjusted to the situation I need, I lock it and store it under a VP-name.
next time I open the drawing, the situation has changed, and the camera is in another position
Why that? I have to unlock the VP and again get to the last situation, again with try&fail. also entering the recorded coordinates doesn´t help, the distance is alwas wrong, and has to be anjusted again and again.

Obviously this is an AutopCAd-Variable Setting, because on another PC with AutoCAD the stored VP stays the same;
or it has something to do with the graphic driver.
Anyone had this problem?

System is:
AutoCAD Civil 3D + NXT
32 bit / Win XP / 2 GB / Nvidia Quadro FX 1700

Thanks in advance

Thomas