"invalid memory alloc request size" + "Could not open file "pg_clog/XXXX"

Hi

In my Postgresql 8.4.0 server, since this morning some tables are
unavailable, see example below :

--> pg_dump MY_DB > bkp_MY_DB.dmp
pg_dump: SQL command failed
pg_dump: Error message from server: ERROR: invalid memory alloc request
size 18446744073709551613
pg_dump: The command was: COPY <schema>.<unavailable_table> (col1, col2,
...).

--> vacuum analyze <schema>.<unavailable_table> ;
WARNING: terminating connection because of crash of another
server process
DETAIL: The postmaster has commanded this server process to
roll back the current transaction and exit, because another server process
exited abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the
database and repeat your command.

--> select * from <schema>.<unavailable_table> ;
ERROR: invalid memory alloc request size 18446744073709551613

--> server log file
Feb 29 05:31:44 my_server postgres[6686]: [17-1] user=,db= LOG: server
process (PID 3887) was terminated by signal 11: Segmentation fault
Feb 29 05:31:44 my_server postgres[6686]: [18-1] user=,db= LOG:
terminating any other active server processes
Feb 29 05:31:44 my_server postgres[6686]: [19-1] user=,db= LOG: all server
processes terminated; reinitializing
Feb 29 05:31:44 my_server postgres[3892]: [20-1] user=,db= LOG: database
system was interrupted; last known up at 2012-02-29 05:22:33 CET
Feb 29 05:31:44 my_server postgres[3892]: [21-1] user=,db= LOG: database
system was not properly shut down; automatic recovery in progress
Feb 29 05:31:44 my_server postgres[3892]: [22-1] user=,db= LOG: redo
starts at 10/67C2A3B8
Feb 29 05:31:45 my_server postgres[3892]: [23-1] user=,db= LOG: record
with zero length at 10/68BCF990
Feb 29 05:31:45 my_server postgres[3892]: [24-1] user=,db= LOG: redo done
at 10/68BCF960
Feb 29 05:31:45 my_server postgres[3892]: [25-1] user=,db= LOG: last
completed transaction was at log time 2012-02-29 05:31:42.618352+01
Feb 29 05:31:45 my_server postgres[6686]: [20-1] user=,db= LOG: database
system is ready to accept connections
Feb 29 05:32:52 my_server postgres[4469]: [21-1]
user=[unknown],db=[unknown] LOG: incomplete startup packet
Feb 29 05:33:52 my_server postgres[6686]: [21-1] user=,db= LOG: server
process (PID 5151) was terminated by signal 11: Segmentation fault
Feb 29 05:33:52 my_server postgres[6686]: [22-1] user=,db= LOG:
terminating any other active server processes
Feb 29 05:33:52 my_server postgres[6686]: [23-1] user=,db= LOG: all server
processes terminated; reinitializing
Feb 29 05:33:52 my_server postgres[5152]: [24-1] user=,db= LOG: database
system was interrupted; last known up at 2012-02-29 05:31:45 CET
Feb 29 05:33:52 my_server postgres[5152]: [25-1] user=,db= LOG: database
system was not properly shut down; automatic recovery in progress
Feb 29 05:33:52 my_server postgres[5152]: [26-1] user=,db= LOG: record
with zero length at 10/68BCF9D8
Feb 29 05:33:52 my_server postgres[5152]: [27-1] user=,db= LOG: redo is
not required
Feb 29 05:33:52 my_server postgres[5153]: [24-1] user=match,db=MY_DB
FATAL: the database system is in recovery mode
Feb 29 05:33:52 my_server postgres[6686]: [24-1] user=,db= LOG: database
system is ready to accept connections
Feb 29 05:37:19 my_server postgres[6686]: [25-1] user=,db= LOG: server
process (PID 8065) was terminated by signal 11: Segmentation fault
Feb 29 05:37:19 my_server postgres[6686]: [26-1] user=,db= LOG:
terminating any other active server processes
Feb 29 05:37:19 my_server postgres[6686]: [27-1] user=,db= LOG: all server
processes terminated; reinitializing
Feb 29 05:37:19 my_server postgres[8066]: [28-1] user=,db= LOG: database
system was interrupted; last known up at 2012-02-29 05:33:52 CET
Feb 29 05:37:19 my_server postgres[8066]: [29-1] user=,db= LOG: database
system was not properly shut down; automatic recovery in progress
Feb 29 05:37:19 my_server postgres[8066]: [30-1] user=,db= LOG: redo
starts at 10/68BCFA20
Feb 29 05:37:19 my_server postgres[8066]: [31-1] user=,db= LOG: record
with zero length at 10/68BD5BD0
Feb 29 05:37:19 my_server postgres[8066]: [32-1] user=,db= LOG: redo done
at 10/68BD5BA0
Feb 29 05:37:19 my_server postgres[8066]: [33-1] user=,db= LOG: last
completed transaction was at log time 2012-02-29 05:35:44.468968+01
Feb 29 05:37:19 my_server postgres[6686]: [28-1] user=,db= LOG: database
system is ready to accept connections
Feb 29 05:38:27 my_server postgres[8639]: [29-1]
user=[unknown],db=[unknown] LOG: incomplete startup packet
Feb 29 05:38:53 my_server postgres[6686]: [29-1] user=,db= LOG: server
process (PID 8809) was terminated by signal 11: Segmentation fault

I have tried to restart Postgresql but it did not solve these issues
I cannot backup the full database because some tables have become unreadable
I have got 7 databases on this server and only 2 have got this problem

What could be the cause of the problem ?
Is there a way to fix it without losing data and without dropping and
recreating the db with my nightly pg_dump backup ?

Thank you by advance for your help