User manual says
A9 = "Start of Setup"
AA = "System has transitioned into ACPI mode, Interrupt controller is in APIC mode"
Old system like that you can't go wrong replacing the onboard CR2032 battery ... it can cause all sorts of strange and apparently unrelated problems if it's dead/dying.
I think the machine is halting just after initializing the PCIe bus. Try pushing the GPU card into the slot, or try removing then remounting it, electrical contacts may need to be cleaned. If it's not a GPU card issue then it could be CPU having uneven mounting pressure.
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[/Korth]
