Hello,
We just had the exact same problem on a Dell PowerEdge R610 equipped with a Dell PERC H700 after an upgrade from ESXi 4.1 to ESXi 5.1u1. The problem was caused by the version 12.10.3-0001 of the RAID controller, that reports some false alarms about multibit ECC errors. The issue was corrected by upgrading the firmware of the RAID controller to the latest version: 12.10.4-0001 (A10). You can download the firmware upgrade here: Driver Details | Dell US. As you can see in the release notes, this version "Fixes an issue where seemingly random multibit ECC errors are seen".