DTM problem

[Jack G]

AutoCAD 2000 SP2, SurvCADD CES March or June 2001, Cogo & DTM

I have not been able to nail down the exact sequence of events, but on some
files (not all), after I have contoured the dwg in DTM, I go back to the
Cogo menu to alter the handles of the contour lines. At some point in time,
all of the 3d objects will disappear from the screen. Sometimes they will
re-appear, after turning on a layer (never the same layer), but you cannot
pick them and they will not plot. Autodesk says the database has been
corrupted. What have I done to corrupt the database? I am a fairly
experienced Acad (10 years) & SurvCADD Cogo (2 years) user, and have only
seen this happen with 3d contours in the DTM module (2d polylines with
elevations). Audit will sometimes (not always) show a layer conflict but
does not tell me what the conflict is. MATCHING LAYER BLAH BLAH. Other
people in my office who use AutoCAD and TerraModel cannot use these files
(similar to a Proxy problem). I have already done the "Block it out when
everything is showing and working" trick, and sometimes this works, but not
always. Anyone else having trouble with the SurvCADD CES DTM module in this
manner?