[G4] G4 DA shuts down, console gives PMU clue

[MnVikes]

I purchased (via Ebay) a used G4/466 DA a few weeks ago. I've noticed that
when the computer has been running several hours (mostly when its
unattended) it will shut itself down. At first I was racking my brain to
remember whether I shut it down and forgot, but I've come to the realization
that its not MY malfunction! :)

I've checked the system console logs and after the shut down the system log
looks something like the following:

Apr  9 06:51:27 Mitch mDNSResponder[146]: mDNSResponder Sleeping at 18277187
Dec 31 19:00:02 localhost syslogd: restart
Dec 31 19:00:02 localhost syslogd: kernel boot file is /mach_kernel
Dec 31 19:00:02 localhost kernel: standard timeslicing quantum is 10000 us
Dec 31 19:00:02 localhost kernel: vm_page_bootstrap: 45400 free pages
Dec 31 19:00:02 localhost kernel: mig_table_max_displ = 68
Dec 31 19:00:02 localhost kernel: IOKit Component Version 7.8:
Dec 31 19:00:02 localhost kernel: Wed Dec 22 14:23:55 PST 2004;
root(rcbuilder):RELEASE_PPC/iokit/RELEASE
Dec 31 19:00:02 localhost kernel: 69 prelinked modules
Dec 31 19:00:02 localhost kernel: Copyright (c) 1982, 1986, 1989, 1991, 1993
Dec 31 19:00:02 localhost kernel: The Regents of the University of
California. All rights reserved.
Dec 31 19:00:03 localhost kernel:
Dec 31 19:00:03 localhost kernel: using 491 buffer headers and 491 cluster
IO buffer headers
Dec 31 19:00:03 localhost kernel: Adaptec warning: Resetting SCSI bus.
Dec 31 19:00:03 localhost kernel: ApplePMU::PMU FORCED SHUTDOWN, CAUSE =
-122
Dec 31 19:00:03 localhost kernel: ApplePMU::CLOCK RESET!  PMU WAS PROBABLY
RESET SOMEHOW!!
Dec 31 19:00:03 localhost kernel: Security auditing service present
Dec 31 19:00:03 localhost kernel: BSM auditing present
Dec 31 19:00:03 localhost kernel: From path:
"/pci at f2000000/mac-io at 17/ata-4 at 1f000/@0:10,\mach_kernel", Waiting on <dict
ID="0"><key>IOPathMatch</key><string
ID="1">IODeviceTree:/pci at f2000000/mac-io at 17/ata-4 at 1f000/@0:10</string></dict
>
Dec 31 19:00:03 localhost kernel: FWOHCI Error trying to clear PHY ports.
Dec 31 19:00:03 localhost kernel: Got boot device =
IOService:/MacRISC2PE/pci at f2000000/AppleMacRiscPCI/mac-io at 17/AppleKeyLargo/a
ta-4 at 1f000/KeyLargoATA/ATADeviceNub at 0/IOATABlockStorageDriver/IOATABlockStor
ageDevice/IOBlockStorageDriver/WDC WD800JB-00JJA0
Media/IOApplePartitionScheme/Apple_HFS_Untitled_2 at 10
Dec 31 19:00:03 localhost kernel: BSD root: disk0s10, major 14, minor 10
Dec 31 19:00:03 localhost kernel: jnl: replay_journal: from: 4976640 to:
7681536 (joffset 0x82b000)
Dec 31 19:00:03 localhost kernel: FWOHCI Error trying to clear PHY ports.
Dec 31 19:00:03 localhost kernel: Local FireWire GUID = 0x3065ff:0xfeed1a76
Dec 31 19:00:03 localhost kernel: WARNING: preposterous time in Real Time
Clock -- CHECK AND RESET THE DATE!
Dec 31 19:00:03 localhost kernel: Jettisoning kernel linker.
Dec 31 19:00:03 localhost kextd[88]: registering service
"com.apple.KernelExtensionServer"
Dec 31 19:00:04 localhost kernel: Resetting IOCatalogue.
Dec 31 19:00:10 localhost kernel: Matching service count = 0
Dec 31 19:00:10 localhost kernel: AppleRS232Serial:        0        0
AppleRS232Serial::start - returning false early, Connector or machine
incorrect
Dec 31 19:00:10 localhost kernel: ROM ndrv for ATY,Rage128Pd is too old
(0xb6246498)
Dec 31 19:00:10 localhost kernel: AirPortFirmware: start Sta f/w download
Dec 31 19:00:10 localhost kernel: obtaining ID
Dec 31 19:00:10 localhost kernel: from Registry
Dec 31 19:00:10 localhost kernel: ATIRage128: using AGP


As you might have noticed, the system clock gets reset to a "preposterous"
time and date when the system reboots, and the log indicates that the PMU
caused a forced shutdown. I've hit the PMU reset button (not this time, even
though the log indicates that I did!), and I've checked the battery's
voltage (OK). The only other home remedy I can think of is a surge
protector, but Consolidated Edison of New York has NEVER caused any of my
other machines such harm, so I'm doubting they are the cause.
I'm asking if anyone has any further information or can estimate the cost of
the repair and how to obtain it. This is very frustrating and I really don't
relish bringing the issue up to the Ebay seller (it was a Money Order
transaction so I'm possibly SOL anyway).