I just necroed a 2 year old thread about the same issue.

X58 motherboards not showing all RAM using the old i9xx CPUs. OK I just repasted my old X58 and locked down the cooler on the CPU, and now…

The Old X58 Motherboard “RAM Not Showing” Problem – Also on an older thread.

Source: [H]ardOCP – The Old X58 Motherboard “RAM Not Showing” Problem – Also on an older thread.