Investigate your SAN performance. We have experienced this exact situation on a Virtualized SQL Server where it appeared that Data, Log, TempDb and Backups were on separate LUNs, but the...

A long shot, but make sure your Networks (i.e. friendly name for NICs in 'Network Connections') are named EXACTLY the same on each Node, e.g. "Public', "Heatbeat1" etc. Even a...