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
anotherserver 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'swhy 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