Suspect a power supply problem if you experience any of the following symptoms, particularly in combination:
Parity check errors. Such errors may be caused by defective or poorly seated memory or by overheating, but insufficient or poorly regulated +3.3VDC or +5VDC (depending on memory type) from a failing or inadequate power supply is a likely cause.
Sporadic or regular boot failures. Obviously, such errors may instead be caused by hard disk, cable, or disk controller problems, but inadequate or poorly regulated +12VDC (less commonly, +5VDC) is also a common cause of this problem.
Spontaneous reboots or system lockups during routine operations, not attributable to running a particular program. Numerous other factors can cause this problem, but one common cause is insufficient or poorly regulated +3.3VDC and/or +5VDC being provided to the memory and/or processor.
Lockups after installing a new processor, memory, disk drive, or expansion card. Driver issues and resource conflicts aside, this problem commonly occurs when new components overload a marginal power supply. This is particularly likely to occur if you make dramatic changes to the system, such as replacing a slow CPU with a fast, high-current CPU; if you expand memory significantly, e.g., from 32 MB to 128 MB; if you add a high-current expansion card such as a fast AGP video card or internal modem; or if you add a high-current drive such as a high-performance SCSI hard disk or a CD burner ...