[sudo-users] sudo su terminates the shell immediately after the first keystroke

[No Name]

Hello all,
I have a bizzare problem with sudo that's been working fine for years.
Obviously something changed recenly but I don't know what. On our linux
machines when I do sudo su -  I get the root shell as expected but as soon
as I type any character it terminates the shell immediately.

[/etc] $ sudo su -
# i               <--- was trying to type 'id' but as soon as I pressed i it
terminated the root shell
logout
[/etc] $

I get the same behavior when I do sudo su - <user>. However, su by itself
works fine. Our sudoers file is served over NFS, at first I was suspecting
it was related to the NFS exports but it looks OK and if it wasn't I'm sure
sudo would complain that it couldn't read the file etc. So I'm completely
lost here, any ideas what might be causing this or if there's a way to debug
it? My only thought at this time is perhaps something to do with the way
sudo spawns /dev/pts.

Thanks