Mapped Drive automatically get disconnect

there is one problem in which after logining to domain users can
able to access network drive for some time but after 1 hr or 2 hr the
> > network drive gets disconnected automatically and the user computer

My server is not restarted or shutdown , on the same time the othere
users can able to access the
common drive from server , to connect again to server we have make
logoff & login again , then user can able to connect to common drive,
on the same time when my common drive is disconnected i m getting ping
reply.
there must be some different cause