Random server overload

Hello,

We are experiencing database random overloads caused by IDLE processes.
Their count jumps from normal ~70 connections to 250-300 with high I/O
(30-40% wa, when normal ~ 1 % wa).

The overload isn't long and lasts about 5 -10 minutes just a couple of
times during the month.

Please suggest how to debug this issue and find the cause of the
overloads. Or mby we should tune our config file ?

errorlog example:
2013-09-30 10:37:45 EEST FATAL: sorry, too many clients already
2013-09-30 10:37:45 EEST FATAL: remaining connection slots are reserved
for non-replication superuser connections
...

config file:

max_connections = 250
shared_buffers = 16GB
temp_buffers = 16MB
max_prepared_transactions = 0
work_mem = 448MB
maintenance_work_mem = 4GB
max_stack_depth = 6MB
wal_buffers = 18MB
checkpoint_segments = 30
checkpoint_timeout = 5min
checkpoint_warning = 30s
random_page_cost = 4.0
cpu_tuple_cost = 0.01
cpu_index_tuple_cost = 0.005
effective_cache_size = 50GB
default_statistics_target = 100
autovacuum = on

othr values are defaults.

System:
RAM 74 GB
PostgreSQL 9.1.9, Debian 6
Database size on disc: 84 GB data + 23 GB indexes. Different LVMs on
RAID 10.

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Best regards