7.3 crash

I'm running Red Hat 7.3 on several client servers. Two of the machines have locked up for no apparent reason. One machine in particular will lock up over-night and has done so numerous times but without consistency. You can't even log in at the master. Re-booting takes care of this. I have checked for power surges, replaced the hard drive, checked for rootkits... What else should I check. The logs show nothing. Is this hardware related or software?