type flatten in the command line

select your schizofrenic polylines and hit enter, when it asks "remove hidden lines" say no (u can delete later if you find some)

still giving you problems?

type pe then enter, then m for multiple (if this is the case) choose the pshychotic lines, then type l for for lt poly lt scale and turn on

if your still having problems, it is most likely you have 3d polylines which cannot contain arcs, or linetypes, explode and rejoin or redraw