This can be caused by bad hardware (memory, motherboard, etc.). Try running a memory-testing program on your PC. Note that, even though every memory testing program you try will report your memory as being fine, it's possible for slightly marginal memory to pass all memory tests, yet fail under operating conditions (such as during bus mastering DMA from a SCSI controller like the Adaptec 1542, when you're beating on memory by compiling a kernel, or just when the system's running particularly hot).
The SIG11 FAQ (listed below) points up slow memory as being the most common problem. Increase the number of wait states in your BIOS setup, or get faster memory.
For me the guilty party has been bad cache RAM or a bad on-board cache controller. Try disabling the on-board (secondary) cache in the BIOS setup and see if that solves the problem.
There's an extensive FAQ on this at the SIG11 problem FAQ