Cannot login for short period of time

Greets,

Pg: 8.3.7

I'm trying to diagnose why I cannot login to Pg on occasion. The psql
command will just hang (so I cannot get in to see what it's doing) and
a telnet into 5432 will give the usual:

Connected to localhost (127.0.0.1).
Escape character is '^]'.

indicating the backend accepting the connection, but not proceeding.

The number of connections at the time are small, file descriptor usage
will be small, etc, server load low.

It will then spontaneously recover as if it's just completed doing
something IO intensive.

My config:

listen_addresses = '1.2.1.1,127.0.0.1'
port = 5432
max_connections = 2048
shared_buffers = 520MB
temp_buffers = 128MB
work_mem = 1512MB
max_fsm_pages = 512000
fsync = off
checkpoint_segments = 32
effective_cache_size = 512MB
constraint_exclusion = on
logging_collector = on
track_counts = on
autovacuum = on
autovacuum_freeze_max_age = 2000000000

I'm rerunning things with statement logging turned on to get a clue.
Incidentally, I did have to reindex the system tables in single-user
mode (got a "Cannot find namespace 0" error at some point).

Anyone have an idea what this strange connectivity-delay could be about?

Cheers
Henry