Problem solved by changing to InnoDB as storage engine for mySQL and using the following settings:
CACHES AND LIMITS
tmp-table-size = 128M
max-heap-table-size = 128M
query-cache-type = 1
query-cache-size = 128M
max-connections = 500
thread-cache-size = 35
open-files-limit = 65535
table-definition-cache = 4096
table-open-cache = 4096
INNODB
innodb-flush-method = O_DIRECT
innodb-log-files-in-group = 2
innodb-log-file-size = 32M
innodb-flush-log-at-trx-commit = 1
innodb-file-per-table = 1
innodb_file_per_table = 1