server gets slow at certain time

I am using Postgresql 8.1 in 8-cpu cores with 32 gb ram with CestOS 5.
I have different DBs in the system and the large one is 12 GB and it is a
CRM DB and heavily used.
Every day after 1 pm the system gets slow and after analyzing the nmon
spreadsheet, I found that IO spike at that period.
Users write data much at that period of time.
DB parameters are as follows:
max_connections= 500
shared_buffers=122144
effective_cache_size = 1835010
work_mem = 8192
max_fsm_pages= 356656
fsync=on
commit_delay=0
commit_siblings= 5
random_page_cost = 0.01
default_statistics_target=100
wah_buffers = 500

Any idea please how to find the bottleneck and improve performance.
Let me know if any more information is needed.