PostgreSQL

Hi
I have some problems with the PostgreSQL 8.3.6.
The client(Microsoft Access 2000) link postgresql table(via ODBC) and
work with this. Sometimes on the client appear:
ODBC--call failed.
Could not send Query(connection dead)(#26);

In PostgreSQL log appear:
could not receive data from client: Connection timed out
unexpected EOF on client connection

In the Client (Microsoft Access 2000) log :
STATEMENT ERROR: func=SC_execute, desc='', errnum=27, sqlstate=,
errmsg='Could not send query to backend'

I want to mention that the problem does not appear on the PostgreSQL8.1.

We couldn't find when it happens, and why..

postgresql.conf:
max_connections = 256
tcp_keepalives_idle = 1 # TCP_KEEPIDLE, in seconds;
# 0 selects the system default
tcp_keepalives_interval = 1 # TCP_KEEPINTVL, in seconds;
# 0 selects the system default
tcp_keepalives_count = 1 # TCP_KEEPCNT;
shared_buffers = 512MB
work_mem = 32MB # min 64kB
maintenance_work_mem = 256MB # min 1MB
max_fsm_pages = 262145 # min max_fsm_relations*16, 6
bytes each
# (change requires restart)
max_fsm_relations = 16384 # min 100, ~70 bytes each
wal_buffers = 256kB # min 32kB
checkpoint_segments = 128 # in logfile segments, min 1,
16MB each

Can any one help me with this ?