Hello and welcome to the TSF, I don't see the 6750 as supported on the Asus website for your mobo, have you flashed the bios to the latest revision. The latest BIOS for your motherboard is 0213
You can get the latest BIOS here:
http://support.asus.com/download/dow...&model=P5LD2-X
CPU support by BIOS is located here:
http://support.asus.com/cpusupport/c...&model=P5LD2-X
I am not seeing 1333 support for your motherboard, unless you can manually overclock it to 1333:
http://www.asus.com.tw/products.aspx...50&modelmenu=1