Older PC stands-by only momentarily

I have an older PC with limited acpi capabilities and 2.6.17-1.2141_FC4 Code:

CPU type : AMD Athlon(tm)
Processor CPU speed : 856.102 MHz
Cache size : 512 KB Bogomips : 1714.25
Processor ID : 0
Bus mastering control : no
Power management : no
Throttling control : no
Limit interface : no
Active C-state : C1
C-states (incl. C0) : 2

But it does have:

ACPI: (supports S0 S1 S5) and /sys/power/state == "standby disk".

If I do a echo -n "standby" >state, then it appears to suspend for 1-3 seconds, power light blinking, etc. Then comes back to life. I cannot figure out what is waking it up. acpitool say all wakeup devices are disabled.

# acpitool -w
 Device Sleep state Status
---------------------------------------
 1. PCI0 1 disabled
 2. KBC 1 disabled
 3. USB0 1 disabled
 4. USB1 1 disabled

Here is the system log. The acpi log has nothing in it. Code:

Jul 20 16:37:26 walden4 kernel: Stopping tasks: ================================================== ================================================== ===|
Jul 20 16:37:26 walden4 kernel: pnp: Device 00:0b disabled.
Jul 20 16:37:26 walden4 kernel: pnp: Device 00:0a disabled.
Jul 20 16:37:26 walden4 kernel: ACPI: PCI interrupt for device0000:00:0f.0 disabled
Jul 20 16:37:26 walden4 kernel: ACPI: PCI interrupt for device 0000:00:04.2 disabled
Jul 20 16:37:26 walden4 kernel: ACPI: PCI Interrupt 0000:00:04.2[D] -> Link [LNKD] -> GSI 9 (level, low) -> IRQ 9
Jul 20 16:37:26 walden4 kernel: bttv0: reset, reinitialize
Jul 20 16:37:26 walden4 kernel: bttv0: PLL: 28636363 => 35468950 .. ok Jul 20 16:37:26 walden4 kernel: ACPI: PCI Interrupt 0000:00:0d.1[A] -> Link [LNKA] -> GSI 11 (level, low) -> IRQ 11
Jul 20 16:37:26 walden4 kernel: ACPI: PCI Interrupt 0000:00:0f.0[A] -> Link [LNKC] -> GSI 10 (level, low) -> IRQ 10
Jul 20 16:37:27 walden4 kernel: eth0: link up, 100Mbps, full-duplex, lpa 0x45E1
Jul 20 16:37:27 walden4 kernel: pnp: Device 00:06 does not support activation.
Jul 20 16:37:27 walden4 kernel: pnp: Device 00:07 does not support activation.
Jul 20 16:37:27 walden4 kernel: pnp: Device 00:0a activated.
Jul 20 16:37:27 walden4 kernel: pnp: Device 00:0b activated.
Jul 20 16:37:27 walden4 kernel: Restarting tasks... done