postgres crashes

Hello

I've been installed Postgres 7.4.2 on FreeBSD system and when load
average grow up i've error in postgres like this:
ERROR: permission denied for function varchar
ERROR: permission denied for function varchar

and then postgres crashes.

LOG: server process (PID 9787) was terminated by signal 11
LOG: terminating any other active server processes
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.
LOG: all server processes terminated; reinitializing
FATAL: the database system is starting up

This error occurs on Postgres version 7.4.x .
This error doesn't occur on Postgres version 7.3.x but are much more slower (even 300% slower) then version 7.4.x. and that's why 7.4.x is necessary.

Postgres configuration:
connection 50
shared_buffers 1000

In FreeBSD kernel i've increased SEMMNS and SEMMNI parameters

Can anyone help me, and tell me where is the source of problem: postgres or FreeBSD or both?
Thanks in advance.

Adam Kempa