multi-threading access problem

hello,everyone.
I have a c test-program for "select query",if the program create
thread every 3ms,it runs smoothly.but when the duration time interval reduce
to 2.8ms,the sleeping process increase more and more,and then the server
crash.the cpu and memory used percentage under 50%.
my postgresql.conf file as bellow:

max_connections = 5000
shared_buffers = 1000MB
work_mem = 256MB
maintenance_work_mem = 512MB
max_stack_depth = 10MB
max_fsm_pages = 204800
max_fsm_relations = 1000
checkpoint_segments = 5
checkpoint_timeout = 5min
checkpoint_completion_target = 0.5
checkpoint_warning = 30s
enable_seqscan = off
seq_page_cost = 10.0 # measured on an arbitrary scale
random_page_cost = 20.0 # same scale as above
cpu_tuple_cost = 1.0 # same scale as above
cpu_index_tuple_cost = 0.4 # same scale as above
cpu_operator_cost = 0.2 # same scale as above
effective_cache_size = 128MB
geqo = on
geqo_threshold = 11
geqo_effort = 6
default_statistics_target = 250
from_collapse_limit = 16
join_collapse_limit = 16
datestyle = 'iso, mdy'
lc_messages = 'en_US.UTF-8'
lc_monetary = 'en_US.UTF-8' # locale for monetary formatting
lc_numeric = 'en_US.UTF-8' # locale for number formatting
lc_time = 'en_US.UTF-8'
default_text_search_config = 'pg_catalog.english'
deadlock_timeout = 1s
max_locks_per_transaction = 96

server hardware : double cpu xeon 5405,4G memory.the table of database have
2millions records.

Maybe I should adjust system performance,but how? anyone can help me?
thanks.

Tony Liao