Constantly losing connections to mounted network drives

[Rich Stanton <stantonrj@Cf.ac.uk>]

I am running debian stable ppc on an old mac G3 laptop.  I have 2
mounted network drives - one mounted via samba is on a macOSX panther
system, and the other a netware filestore mounted via ncpfs.  Here is my
relevant fstab entries:

//193.x.x.x/YellowDog  /mnt/mic smbfs username=xxxx,password=xxxx

ServerName/user.name       /mnt/SDrive     ncp
defaults,volume=xxx/xxx,ipserver=dns.hostname,server=ServerName,user=user.name,passwd=xxx,multiple
      0       0

Both volumes mount fine initially however after a while (a few hours in
the case of the netware drive, a day or 2 in the case of the OSX drive)
I seem to lose the connection - if I try & 'ls' in the mounted
directories, I get

ls: reading directory .: Input/output error

If I umount & then mount the drives, everything is fine again.  However
I'm having to remount them several times a day!  I have looked through
/var/log but couldn't find anything relevant.  The OSX system is
certainly up 24/7, bar the odd reboot, and the no-one who accesses the
netware filestore through windows reports any dropped connections there.

Any ideas what's going on?  Why is the connection dropping, or how do I
find out why it's dropping?  Is there anything I can do (short of
scheduling regular cron jobs to remount them) to ensure they reconnect
if they connection is lost?