Windows MySQL: Out of memory - mysql

Question

We are not able to create a stable configuration of a production server with MySQL and Tomcat Application Server. The MySQL throws very often an error:
MySQL: Out of memory (Needed 429496728 bytes)
It was a Windows Server 2012 with 64 GB of RAM. We see in the process tab, that the MySQLd commited 64 GB of RAM (that means all the available RAM in the server).
For your explaination, the Application uses completly inMemory based Tables in the MySQL Server.
See the my.ini Configuration
[client]
port=3306
[mysql]
default-character-set=utf8
[mysqld]
port=3306
basedir="F:/MySQL Server 5.6/"
datadir="F:/MySQL Server 5.6/data/"
character-set-server=utf8
default-storage-engine=MyISAM
sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"
max_connections=200
query_cache_size=0
# table_cache=256
tmp_table_size=6G
max_heap_table_size=6G
max_tmp_tables=2048
open_files_limit=40000
thread_cache_size=8
myisam_max_sort_file_size=100G
myisam_sort_buffer_size=32M
myisam_use_mmap=0
concurrent_insert=2
key_buffer_size=2G
read_buffer_size=512M
read_rnd_buffer_size=512M
sort_buffer_size=128M
bulk_insert_buffer_size=32M
#skip-innodb
innodb_additional_mem_pool_size=15M
innodb_flush_log_at_trx_commit=1
innodb_log_buffer_size=7M
innodb_buffer_pool_size=686M
innodb_log_file_size=343M
innodb_thread_concurrency=34
slow_query_log=1
log-queries-not-using-indexes=1
long_query_time=1
log-output=FILE,TABLE
slow_query_log_file="F:/MySQL Server 5.6/localhost-slow-query.log"
event-scheduler=ON
Does anybody have a suitable solution to get this fixed?
We also use the MySQL Tuner to get informations, here the results:
What I cannot attach is the MySQL Tuner results as image (too few reputations). But look here.
Up for 39days
Data in Memory Tables 42G (Tables:281)
Data in MyISAM tables 31G (Tables 668)
total buffers: 2.3G global + 144.5M per Thread (200 max threads)
maximum possible memory usage: 30.5G (47% of installed RAM)
Key buffer size /total MyISAM indixes: 640M/7.0G
query cache disabled
joins performed without indixes: 337737