try exporting it as something else a jpeg or tiff and see if it still happens.
if it is still happening try creating a new .mxd
if it is still happening, if you are using a gdb, it might be corrupt and you may have to make a new one. I have had issues like this with curves although I find them before exporting as a result of topolgy runs. The line work should still be good so either load the data from the existing feature classes into the new GDB via catalog or just copy and paste them from ArcMap
or if you have 3D analyst extension with ERDAS we found it makes curves buggy within arcmap.