It could be down to the U3S6 card that you are using as the asus website is showing that this is compatible with the following motherboards ;
Code:
Maximus III Formula
P7P55D Deluxe
P7P55D EVO
P7P55D PRO
P7P55D
P7P55D LE
P7P55 LX
P7P55 WS Supercomputer
P6T Deluxe
P6T Deluxe V2
P6TD deluxe
P6T SE
Rampage II Extreme
Rampage II Gene
M4A79T Deluxe
M4A785TD-V EVO
M4A77TD PRO
M4A79T Deluxe/U3S6
M4A77TD PRO/U3S6
M4A785TD-V EVO/U3S6
I can see that the Asus P5B is not listed and as such this could be the source of the issue.
If you log this with the returns team using the OLQ system Here they will be able to assist you further with this issue.
Best Regards,