Gigabyte 965P-DS3 ignores boot priority?

My Gigabyte 965P-DS3 ignores boot priority. This motherboard uses the standard Award BIOS in which you can set 3 boot device and their order. I set it to (1) USB-HD, (2) CDROM, (3) Harddrisk.
Alternately, at the Gigabyte Startup Screen the following runs across the bottom of the screen:
TAB: Post Screen; DEL: Bios Setip/Q-flash; F9: Xpress recovery 2; F12: Boot Menu; and END: Q-flash
Boot menu (F12) allows changing the boot sequence without entering the BIOS setup.

No matter what I try, it ALWAYS tries to boot my SATA Harddisk first. Thus, I cannot boot the USB-HD (which stores all of my boot utilities). I have to pull the harddrive's power or the SATA cable before booting. To image/restore the HD, I have to put it in an external USB dock. I didn't have to do this before - it just started a few days ago.

I built this computer almost 3 years ago. It has worked good except for warm boot problems (it will NOT warm boot -0 I always have to wait 15 minutes for it to cool down before rebooting). I've always ran this system overclocked, but this problem occurs whether its o/c'ed or not. I have highly sufficient cooling - CPU maxes at about 46 C, system a few degrees higher.

My setup:
MB: Gigabyte 965P-DS3
PS: Antec 520W TruPower (brand-new)
CPU: Intel E2160 @ 1.8GHz (o/c'ed to 3.15GHz at stock voltage)
CPU Cooling: Noctua NC-12 (the big boy)
RAM: Patriot DDR2 800 Mhz (2 x 2GB)
HD: SATA 750GB Seagate
Video: Radeon 5770 (also tried Intel GMA on-board)
Optical: Asus & Pioneer DVRs (both SATA)
OS: Win XP SP3

Things I've tried:
- Safe BIOS settings. No overclocking anything.
- Every combination of SATA cables and SATA ports.
- RAM is OK (low level diagnostics ran)
- Power supply is brand new. (I was having other problems and thought it was power - it wasn't)
- Ran some low-level motherboard diagnostics - everything checks OK (however, can't check SATA as the computer will _not_ boot to anything other than SATA harddisk)
- Disconnected _everything_ (USB headers, audio header, IDE PATA drive) - behavior is unchanged
- Tried various BIOS settings - safe, AHCI (which I've never used) - no change
- Reset BIOS via jumper on motherboard - no change
- Tried Radeon X600 video card - no change
- CMOS battery seems OK - it holds settings after being disconnected from power overnight

I am guessing at this point that the Gigabyte motherboard has just gone bad and developed this unfriendly quirk.
Any help would be greatly appreciated! Thanks!