Freenas iSCSI errors / crash

Hi, we've been running freenas for some time now and on the whole it works well. However we are continually having problems with our VMware host which is using storage from freenas via iSCSI. The symptoms are always the same, guest vm's stop working, datastore offline (ESX5.5), freenas generally unresponsive. The freenas host machine specs are below

Freenas version: 9.2.1.5
Supermicro M/B
Opteron 6272
64GB ECC Ram
IBM1015 HBA crossflashed to LSI
6 x 3TB Seagate Constellation CS SAS
1 x 128gb...

Freenas iSCSI errors / crash