i found out a strange effect in dxf files. if i am working on a dxf, it happens that several objects simply disappear from screen.
If i reopen the dxf, or change any properties of the layer, the objects are visible again (sometimes).
Example: if i have a layer for dimensions and measurements, sometimes the measurements disappear. if i zoom out, the dimensions are shown again; but they are to small to read. if i zoom in again, they are getting invisible again.