RHEL 5.2 is really stable, and SATA is not exactly ultra-modern hardware. It's a good possibility that there is a defect in the motherboard, CPU, or memory. Have you tried booting off the CD-ROM without the hard drive? (Download a "live CD" based off another distribution). Try swapping memory banks to see if the problem occurs at a different point during the process. Have you manually adjusted any BIOS settings? Overriding the BIOS auto-detected memory-refresh settings can have such effects.