WARNING: buffer refcount leak

I'm working on a new machine, and i think it's got possible bad
hardware, since that seems more likely than a bug in postgresql. I'm
wondering if someone has any idea what kind of hardware failure might
cause this message:

WARNING: buffer refcount leak: [424] (freeNext=425, freePrev=423,
rel=0/0, blockNum=4294967295, flags=0x1c, refcount=-631 30464)

The one time this happened, postmaster displayed the refcount leak, or
it would segfault or it crashed with messages like these: (free():
invalid pointer 0xa06ffc0!). Usually it just works fine, this appears
to be a very intermittent problem. We've already replaced the SCA
backplane, the SCSI cables, the RAID controller, and the motherboard.
The only components not replaced are the memory and the CPUs.

I've run Memtest86 on the box for several days without it finding any
bad memory. It's the first test I run on any new machine. Can anyone
recommend any good (free) diagnostics programs like Memtest86 that
check CPUs, PCI bus, etc, etc.

The machine is a dual xeon 2.8, 4gb ECC ram, and 14 15k 36G U320
drives with a megaraid 320-2x controller. running fedora core 1,
postgres 7.3.4

thanks for any advice, and i hope this isn't too off topic.

--brian