CUSL2-C won't do keyboard boot

Hi,

I have a system based on the Asus CUSL2-C board here (without both of
onboard audio and vga) which has the following problem:

If I switch on the system via the power button on the case everything
works as expected, after I shutdown the system but do not cut the power
I can also power on the system via the configured keyboard combination
(Ctrl-Esc).

When the power was cut (e.g. by flipping the power switch of the ATX
power supply on the back), booting by keyboard won't work. Sometimes
even an immediately following boot via the power button on the case
doesn't work properly when I tried Ctrl-Esc before (system puts me in
the BIOS with some warning about core voltage and overclocking).

The CPU is operating at the intended frequency and I have no intention
to overclock the system.

Is there some way I could get the system to boot from keyboard? I have
already tried to switch the power supply - same results as above.
Should I try to disable the overclocking features?

Thanks for any input on this issue.

Thomas Jahns
--
"Computers are good at following instructions,
but not at reading your mind."
D. E. Knuth, The TeXbook, Addison-Wesley 1984, 1986, 1996, p. 9