Magento – Place Order hangs with 2006 MySQL server has gone away

I'm having an issue with placing orders for a store that has taken thousands of orders. I am using Enterprise One Page Checkout with Authorize.net Direct Post

Without anything having changed to my knowledge yesterday Place Order started hanging and generating the following error:

URL: []/authorizenet/directpost_payment/place/form_key/.../
IP Address: ...
Time: 2014-01-31 14:44:16 GMT

Error:
SQLSTATE[HY000]: General error: 2006 MySQL server has gone away

Trace:
0 /var/www/.../lib/Zend/Db/Adapter/Pdo/Abstract.php(305): PDO->beginTransaction()
1 /var/www/.../lib/Zend/Db/Adapter/Abstract.php(495): Zend_Db_Adapter_Pdo_Abstract->_beginTransaction()
2 /var/www/.../lib/Varien/Db/Adapter/Pdo/Mysql.php(219): Zend_Db_Adapter_Abstract->beginTransaction()
3 /var/www/.../app/code/core/Mage/Core/Model/Resource/Abstract.php(76): Varien_Db_Adapter_Pdo_Mysql->beginTransaction()
4 /var/www/.../app/code/core/Mage/Core/Model/Abstract.php(313): Mage_Core_Model_Resource_Abstract->beginTransaction()
5 /var/www/.../app/code/core/Mage/Sales/Model/Quote.php(1966): Mage_Core_Model_Abstract->save()
6 /var/www/.../app/code/core/Mage/Checkout/controllers/OnepageController.php(624): Mage_Sales_Model_Quote->save()
7 /var/www/.../app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Checkout_OnepageController->saveOrderAction()
8 /var/www/.../app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('saveOrder')
9 /var/www/.../app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
10 /var/www/.../app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
11 /var/www/.../app/Mage.php(683): Mage_Core_Model_App->run(Array)
12 /var/www/.../index.php(87): Mage::run('default', 'store')
13 {main}
`

I have read and been told by the hosting company that this could be related to mysql configuration so here is the my.cnf for the db:

    #this is mostly auto generated by a form at perconas web site.

[mysqld]
bind-address=0.0.0.0

# MyISAM #
key-buffer-size                = 32M
myisam-recover                 = FORCE,BACKUP

# SAFETY #
max-allowed-packet             = 32M
max-connect-errors             = 1000000

# CACHES AND LIMITS #
tmp-table-size                 = 32M
max-heap-table-size            = 32M
query-cache-type               = 0
query-cache-size               = 0
max-connections                = 500
thread-cache-size              = 50
open-files-limit               = 65535
table-definition-cache         = 4096
table-open-cache               = 4096

query_cache_type=1
query_cache_size=128M
query_cache_limit=24M
#query_alloc_block_size=128
#query_cache_min_res_unit=128

# INNODB #
innodb-flush-method            = O_DIRECT
innodb-log-files-in-group      = 2
innodb-log-file-size           = 512M
innodb-flush-log-at-trx-commit = 1
innodb-file-per-table          = 1
innodb-buffer-pool-size        = 26G



# BINARY LOGGING #
log-bin                        = /var/lib/mysql/mysql-bin
expire-logs-days               = 14
sync-binlog                    = 1

# LOGGING #
log-error                      = /var/lib/mysql/mysql-error.log
log-queries-not-using-indexes  = 1
slow-query-log                 = 1
slow-query-log-file            = /var/lib/mysql/mysql-slow.log

innodb_data_file_path=ibdata1:1G;ibdata2:512M:autoextend
innodb_autoextend_increment=512

#max_connections=110
wait_timeout=120
interactive_timeout=120

# modify isolation method for faster inserting.
# Do not uncomment the line below unless you understand what this does.
# transaction-isolation = READ-COMMITTED
# do not reverse lookup clients
skip-name-resolve

#long_query_time=6
#log_slow_queries=/var/log/mysqld-slow.log
#log_queries_not_using_indexes=On
#log_slow_admin_statements=On

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

[mysqld_safe]
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid

Any suggestions or information to help me debug this issue would be greatly appreciated. Thanks.