Exactly the same problem here, but it started after a memory upgrade last saturday.
The memory was replaced twice by the datacenter, and now they think that they are not the problem.
I'm not sure if the problem was caused by this upgrade (messing with memory, cables etc) or if it started just because the server was rebooted (just some update that was not completely active yet perhaps?).
The datacenter will replace some SAS cables and maybe the Raid controller tonight.
These are some logs from "dmesg" command in ESXi SSH:
This is the hardware information:
Motherboard SuperMicro X8DTU-F_R2 Intel Xeon HexCore DualProc [2Proc]
Processor Intel Xeon-Westmere 5620-Quadcore [2.4GHz] Hardware upgrade
Processor Intel Xeon-Westmere 5620-Quadcore [2.4GHz] Hardware upgrade
RAM slot 1 Hynix 8GB DDR3 2Rx4 8GB DDR3 2Rx4 [8GB] Hardware upgrade
RAM slot 2 Hynix 8GB DDR3 2Rx4 8GB DDR3 2Rx4 [8GB] Hardware upgrade
RAM slot 3 Hynix 8GB DDR3 2Rx4 8GB DDR3 2Rx4 [8GB] Hardware upgrade
RAM slot 4 Hynix 8GB DDR3 2Rx4 8GB DDR3 2Rx4 [8GB] Hardware upgrade
RAM slot 5 Hynix 8GB DDR3 2Rx4 8GB DDR3 2Rx4 [8GB] Hardware upgrade
RAM slot 6 Hynix 8GB DDR3 2Rx4 8GB DDR3 2Rx4 [8GB] Hardware upgrade
Drive Controller 0b321170a3f Adaptec \ 5805 Z \ SATA/SAS RAID Hardware upgrade
Battery Adaptec Super Capacitor ZMM-100CC
Hard Drive 1 6sl0kn81 Seagate Cheetah ST3600057SS [600GB] Hardware upgrade
Hard Drive 2 6sl0k531 Seagate Cheetah ST3600057SS [600GB] Hardware upgrade
Hard Drive 3 6sl0k58t Seagate Cheetah ST3600057SS [600GB] Hardware upgrade
Hard Drive 4 z291byjb Seagate ConstellationES.2 ST33000650NS [3000GB] Hardware upgrade
Remote Mgmt Card SuperMicro Winbond WPCM450 - Onboard IPMI-KVM
Network Card SuperMicro AOC-PG-i2+ SuperMicro Gigabit Port
Power Supply SuperMicro PWS-561-1H20 (20) R6.1 560W
Backplane SuperMicro BPN-SAS-815TQ 4 Port Passive
The server was running ESXi 5/5.1 for almost a year without problems before this memory upgrade.