!!! Urgent: btree: lost page

I manage an ISP users with postgres 6.3 on REdhat linux 6.1
recently i have seen some problem in my database.
when i wanna update one of my tables for a special user, ( user is a
field in my Database ), i get this error:

"FATAL 1:
btree: lost page in the chain of duplicates"

this occur for one 3 user of 1000 only. !!!
also, user is a primary key so there is an INDEX for this field in
database.
after that, all the connections to postgres will be disabled. and i
get this message :

"
db do faild: PQexec() -- request was sent to backend but backend
closed the channel before responding.
this probably means that the backend terminated abnormally befor or while
processing the request.
DBD::Pg::db do failed: PQexec() --there is no connection to bachend.
"
plz send ur comments & hints to me.

sincerely
mehdi roomi
roomi@safineh.net