High mysql throughput

on a big server with many user and website from 11AM - 00AM our mysql throughput reaised in max (60 - 80M) this timestamp our customer website load slowing .

we have,'t any Disk IO , and server HDD : 6 * 600 G SAS 15K + 64 G RAM

mysql config :

innodb_buffer_pool_size=8192M (2 G above mysqltuner recommendation)

Screen attach : http://ift.tt/1D8cl7H

Any idea to solve problem ? thanks

---

This comment add after first answer :

Hello Dear Damiano : Thank you so much your answer . about you comment : I will attach completley svreen from server monitor here : http://ift.tt/1vIcuJ6

about slow query : I done this , but every 24 hrs we have under 500 slow query . and most them not occur repeatly .

an other thing , this server big and we have High trafffic web site on it , so I can not ask customer don't use server resource .

mysql> SHOW STATUS LIKE 'Qcache%';
+-------------------------+----------+
| Variable_name           | Value    |
+-------------------------+----------+
| Qcache_free_blocks      | 11573    |
| Qcache_free_memory      | 27291504 |
| Qcache_hits             | 83817933 |
| Qcache_inserts          | 36512628 |
| Qcache_lowmem_prunes    | 15633028 |
| Qcache_not_cached       | 4338965  |
| Qcache_queries_in_cache | 29930    |
| Qcache_total_blocks     | 106130   |
+-------------------------+----------+


8 rows in set (0.00 sec)