Hi Lee,
Ok thank your for your link, so nevermind and i think your issue is not on your ESX (but i can be wrong)
But here some informations about physical switch configuration : http://partnerdirect.dell.com/sites/channel/Documents/EqualLogic_iSCSI_Optimization_for_Dell_PowerConnect_Switches_White_Paper.pdf
Portfast should be enable enable only on your inter-switch connection.
Because if you are loosing an ISCSI LUN, its because you are loosing your both paths in the same time. So for me , your switch is over used and it rejected your traffic.
Regards,
Julien;