R715 suddenly crashed

Hi,

suddenly crashed R715  (black screen  and nothing  more)
Last activity was to install replace RAM (few weeks ago)

hardware log  last records ( 10 seconds between these 2 events) before crash  were 

The system board VR PLX PG voltage is outside of range. 

The system board VR PLX PG voltage is outside of range.

Removed ram , Server works normally. Re-Installed.  Server started by failed in a few moments with same symptoms + 1 extra log ently

Mon Dec 14 20:02:17 2015 The system board VR PLX PG voltage is outside of range. 

Mon Dec 14 20:02:37 2015 The system board VR PLX PG voltage is outside of range.
Mon Dec 14 20:02:46 2015 The system board fail-safe voltage is outside of range.

so, at the moment my question what "The system board VR PLX PG voltage is outside of range." means? I am unable to find any info.  

Thanks.