Assuming you've verified that the cables are fine, meaning that they themselves are not defective, try the following:
1. Remove the motherboard from the case, if not already out, and then rebuild your system out of the case.
2. If you're using more than one DIMM, try these configurations with all DIMMs:
[CPU] | _ _ _
[CPU] _ | _ _
[CPU] _ _ | _
[CPU] _ _ _ |
3. Clear CMOS by either removing the CR2032, nickel-sized battery for 5-10 seconds, or by disconnecting the P1 and P4 connectors and moving the CLR_CMOS jumper from the 1_2 position to the 2_3 position for about 5-10 seconds