It could be related to the board short circuiting from underneath.
I will recommend to bring the board out of the chassis, and test it on an antistatic non-conductive surface after CLRTC, and see if that still happens.
Once more thing you can check is to try pressing the [F8] key during POST to bring up the boot menu to see if it is device related. You really don't have to go though all the trouble to enter BIOS configuration screen and test the boot device priority one by one.