Mysql hand machine due to high load average and CPU

Mysql is used by only one application which only fire query periodically. When i restart server,It run smooth for period of time. and then crash the whole server. My my.cnf file [mysqld] datadir=/var/lib/mysql socket=/var/lib/mysql/mysql.sock user=mysql # Disabling symbolic-links is recommended to prevent assorted security risks symbolic-links=0 innodb_file_per_table=1 set-variable = max_connections=200 log-slow-queries=/var/lib/mysql/slow.log safe-show-database query_cache_size=128M join_buffer_size=50M tmp_table_size=128M max_heap_table_size=128M table_cache=10K long_query_time =10 open_files_limit=50000 table_open_cache=20000 table_cache=20000 query_cache_limit=134217728 innodb_buffer_pool_size=900M [mysqld_safe] log-error=/var/log/mysqld.log pid-file=/var/run/mysqld/mysqld.pid