bugs or ? in PostgreSQL 7.1

After compiling and porting database from 7.0.3 to 7.1 perfomance was cool,
but later, when many clients connected to our server and process
transactions (creating documents e.t.c.) PostgreSQL 7.1 say's "ERROR:
Cannot insert a duplicate key into unique index pay_docs_list_pkey", but all
indexes and sequences was OK, and after vacuum, reindexing base and
increasing last number of sequence (defined as "serial") errors was
repeated.
And also, time to time, Postgres die with logs:
Server process (pid ...) exited with status 139 at ... Terminating any
active server processes...
The Postmaster has informed me that some other backend died
abnormally and possibly corrupted shared memory. I have rolled back
the current transaction and am going to terminate your database system
connection and exit. Please reconnect to the database system and repeat
your query.

This error usualy after vacuum small tables on the fly

And in final PostgreSQL 7.1 kill our databse, i.e. can't restart on they
database with error - can't restore pg_xlog records...

Postgresql was compiled with
params --enable-locale --enable-recode --with-maxbackends=1024, data was in
koi8 with recode by charset.conf. Also php4.0.4pl1

What occurs???????????

Sorry my bad english, can answer in russian or english

Andrey Mikuloff