USB keyboard only works intermittently in GRUB 2 on reboots - how to make it work consistently?

USB keyboard only works intermittently in GRUB 2 on reboots - how to make
it work consistently?

Something weird is going on with my keyboard. The keyboard is a
USB-connected Unicomp, which works absolutely flawlessly once the computer
is up and running. However, GRUB 2 (specifically, 1.99-27+deb7u1 as
shipped in Debian Wheezy) isn't quite so happy. The failure modes make
this a little difficult to test, but here's what I've been able to deduce
thus far:
The keyboard status LEDs flash during the POST, so the keyboard is
detected (I also get a report "Detected: ... 1 Keyboard"); I've got maybe
one "no keyboard detected" error from the POST, nowhere near enough to
deduce any pattern to that, and I've been rebooting the system quite a few
times in different ways lately
On a power on from a full power off with the front panel power button,
everything seems to work great
On a reboot (either using reboot or Control+Alt+Delete in a booted system
or in GRUB), the keyboard works in GRUB maybe every other time I try
If I hit the Reset button on the computer to reboot, the keyboard
consistently does not work in GRUB when the boot loader comes back, and
many times does not react to key presses during the POST either
What's even more weird is that I don't recall the computer acting like
this before. I'm hoping it's unrelated, but it seems to have started
acting up right around when I hooked up the UPS to some more things around
my desk, instead of only having hooked it up to the computer and monitor.
(No, the UPS is nowhere near overloaded; the load is reported as being in
the range 7-20% depending on usage; around 15-16% of maximum load when the
computer is up and running normally.)
I've Googled and found the suggestion to load the GRUB 2 modules uhci and
usb_keyboard. I added a GRUB_PRELOAD_MODULES declaration specifying the
two of them to /etc/default/grub and re-ran update-grub (there is now
insmod uhci and insmod usb_keyboard in my /boot/grub/grub.cfg), but that
does not seem to have changed anything.
I've moved the keyboard to another USB port, which does not appear to have
helped. I would try an offboard USB controller if I had one handy.
BIOS/UEFI setup reports legacy USB support and legacy USB 3.0 support as
enabled, and even if it didn't, I don't see why it would sometimes work
and sometimes not, especially when many times the only difference between
the two is which side of a warm reboot it's on.
The motherboard is an ASUS M5A97 Pro with UEFI version 1007 02/10/2012.
I can't think of any other configuration changes I've done that coincide
with when the computer started acting up. The logs do mention an upgrade
of linux-image on Aug 1, and that I reinstalled (it's noted as upgrade,
but the from-version and to-version are identical) grub-pc on Aug 2, but
both of those was a week before I even bought the UPS, and two weeks
before I hooked it up to more than just the computer and monitor (I take
care to not change too many things at once). uname -r reports
3.2.0-4-amd64.
I'm running out of ideas to try. How can I get my USB keyboard to work
consistently in GRUB 2? What else can I check?