MySql Error Can't create a new thread (errno 11).?

I am getting this error at times when I have light traffic, and the server is not overloaded, and memory is available.
mysql_select_db(): Can't create a new thread (errno 11). If you are not out of available memory, you can consult the manual for a possible OS-dependent bug in
Server specification :

Dell poweredge 6650
4 gb RAM
4 Intel XEON 2.7 Ghz

The server runs on Red-hat 7.3
mysql 3.23.55
PHP/4.3.2

/** mysql> show variables; **/


bdb_cache_size 8388600
bdb_max_lock 10000
bdb_shared_data OFF
binlog_cache_size 32768
concurrent_insert ON
datadir /mnt/md0/mysql/
delay_key_write ON
delayed_insert_limit 100
delayed_insert_timeout 300
delayed_queue_size 1000
flush OFF
flush_time 0
have_bdb DISABLED
have_gemini NO
have_innodb YES
have_isam YES
have_raid NO
have_openssl NO
init_file
innodb_additional_mem_pool_size
262144000
innodb_buffer_pool_size 891289600
innodb_data_home_dir /mnt/md0/mysql
innodb_file_io_threads 5
innodb_force_recovery 0
innodb_thread_concurrency 8
innodb_flush_log_at_trx_commit 1
innodb_fast_shutdown ON
innodb_log_buffer_size 8388608
innodb_log_file_size 157286400
innodb_mirrored_log_groups 1
interactive_timeout 60
join_buffer_size 131072
key_buffer_size 891285504
low_priority_updates OFF
lower_case_table_names 0
max_allowed_packet 1048576
max_binlog_cache_size 4294967295
max_binlog_size 1073741824
max_connections 1000
max_connect_errors 10
max_delayed_threads 20
max_heap_table_size 16777216
max_join_size 4294967295
max_sort_length 1024
max_user_connections 0
max_tmp_tables 32
max_write_lock_count 4294967295
net_buffer_length 16384
net_read_timeout 30
net_retry_count 10
net_write_timeout 60
open_files_limit 0
protocol_version 10
record_buffer 8384512
record_rnd_buffer 8384512
query_buffer_size 0
sort_buffer 8388600
sql_mode 0
table_cache 1500
table_type INNODB
thread_cache_size 50
thread_stack 131072
transaction_isolation READ-COMMITTED
timezone CST
tmp_table_size 33554432
tmpdir /tmp/
version 3.23.55-Max-log
wait_timeout 60

thanks