Screen full of ?'s

[Abyss]

One of the Debian 2.1 console windows has lost it's normal font and is 
displaying characters as black on white question marks and other assorted 
graphical characters.

This does not change if one logs out then back in again.  It does not 
change if X is started.  It does not change if "setfont" is used.  The only 
solution so far is to reboot Linux.

It only happens occasionally and I would prefer not rebooting when it happens.

Anyone encountered this before?
Thanks in advance.