From time to time, AutoCAD 2005 seems to reset the SAVETIME variable setting to 0, effectively disabling the AutoSave feature.
This is a known problem with version 2005 - AutoCAD temporarily disables AutoSave during the PUBLISH command. If this command fails, AutoSave is not restored.
As a fix, you can use the following VBA reactor to restore AutoSave after each command:
Private Sub AcadDocument_EndCommand (ByVal CommandName As String)
ThisDrawing.SetVariable "SAVETIME", 10
NB added: reported also in AutoCAD 2006