4 x 6870's
biostar 890FXA
hextec 1050W power supply
2 risers for the gfx cards

stock voltage/clocks
default fan control on ubuntu 11.04

i got 4 rigs running and this particular one always seems to die on me, hard crash 4-48 hours after rebooting.  Problem is it runs for at least a few hours so im never able to catch it int the act. 

Anyone know how to diagnose this problem?  I'm looking for tips/tricks something beyond standard trial and error.. maybe some kind of monitor program, crash log file i dont know about, whatever.