mapping dissappears, why?

I am on a win2k domain and use a logon script to delete previous mapped
drives and map two drive letters to another machine.
Occaisionally for some reason the map dissappears.
I am trying to find both why the write failed but also why the map is
removed.
When an attempt is made to write to the mapped drive, and the drive doesn't
respond, will that remove the mapping? If so is there a way to make it stop
removing it? Is that what a persistent map is? How is persistance written in
a logon script?