Mysql Insert/Update Makes Apache Go Crazy

I run a wordpress blog with apache2+mysql5+php5 in a Debian vps with 1024mb ram (plus swap).

When you read the website everything works quick and smooth, but when you have to add data to the database (edit posts, write posts, write comments) something weird happens.
monitoring the system with the top and I see that, for example, as soon as an article has been posted (already got the "article posted" message in the page) the free ram goes suddenly down (some seconds) from 800mb to 0. Swapping starts and the website stops responding. in the meanwhile cpu "wa" goes high (90%++).

Typing ps aux I can see that is not mysql process's fault, actually it seems more that it's apache to cause this ram hogging, in fact an apache restart brings back tons of free ram.

this is my current my.cnf:

Code:
client]
port= 3306
socket= /var/run/mysqld/mysqld.sock

# Here is entries for some specific programs
# The following values assume you have at least 32M ram

# This was formally known as [safe_mysqld]. Both versions are currently parsed.
[mysqld_safe]
socket= /var/run/mysqld/mysqld.sock
nice= 0

[mysqld]
#
# * Basic Settings
#
user= mysql
pid-file= /var/run/mysqld/mysqld.pid
socket= /var/run/mysqld/mysqld.sock
port= 3306
basedir= /usr
datadir= /var/lib/mysql
tmpdir= /tmp
language= /usr/share/mysql/english
skip-external-locking
#
# Instead of skip-networking the default is now to listen only on
# localhost which is more compatible and is not less secure.
bind-address= 127.0.0.1
#
# * Fine Tuning
#

myisam_sort_buffer_size=64M
max_connect_errors=20
connect_timeout=15
interactive_timeout=100

join_buffer_size=1M
sort_buffer_size=1M
read_buffer_size=1M
bulk_insert_buffer_size=16M

key_buffer= 64M
max_allowed_packet= 16M
thread_stack= 128K
thread_cache_size= 8
max_connections = 600
table_cache = 256
#thread_concurrency = 10
#
# * Query Cache Configuration
#
query_cache_limit = 1M
query_cache_size = 16M
#
# * Logging and Replication
#
# Both location gets rotated by the cronjob.
# Be aware that this log type is a performance killer.
#log= /var/log/mysql/mysql.log
#
# Error logging goes to syslog. This is a Debian improvement :)
#
# Here you can see queries with especially long duration
#log_slow_queries= /var/log/mysql/mysql-slow.log
#long_query_time = 2
#log-queries-not-using-indexes
#
# The following can be used as easy to replay backup logs or for replication.
#server-id= 1
#log_bin= /var/log/mysql/mysql-bin.log
# WARNING: Using expire_logs_days without bin_log crashes the server! See README.Debian!
#expire_logs_days= 10
#max_binlog_size = 100M
#binlog_do_db= include_database_name
#binlog_ignore_db= include_database_name
#
# * BerkeleyDB
#
# Using BerkeleyDB is now discouraged as its support will cease in 5.1.12.
skip-bdb
#
# * InnoDB
#
# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
# Read the manual for more InnoDB related options. There are many!
# You might want to disable InnoDB to shrink the mysqld process by circa 100MB.
skip-innodb
#
# * Security Features
#
# Read the manual, too, if you want chroot!
# chroot = /var/lib/mysql/
#
# For generating SSL certificates I recommend the OpenSSL GUI "tinyca".
#
# ssl-ca=/etc/mysql/cacert.pem
# ssl-cert=/etc/mysql/server-cert.pem
# ssl-key=/etc/mysql/server-key.pem