Asus P8Z68-V LX RAM channel not working

I recently encountered a memory issue with my 1 year old assembled PC. For one year, it worked perfectly with not a single hiccup for both intensive gaming sessions and general internet, video and work usage.

It has 2 x 4GB of Kingston 1333MHz ram installed on the usual dual channel setup.

Last week, while playing far cry 3, it crashed unexpectedly and restarted. However, it didnt manage to get past POST. I realised that the red DRAM error light was switched on, so the first thing I did was to switch the ram from the blue slots to the black slots.

This fixed the problem for a few days, before the same thing happened on Borderlands 2. It crashed and couldnt POST. I took out the memory and switched it back to the blue slots. This fixed the issue for a few days before crashing in Borderlands 2 again.

This time i did a memtest on each stick running a few passes on slot B2. No error turned up.

Suspecting it might be the ram slots, I tested the ram on every slot. Turns out that the Channel A1 and A2 (closer to the CPU) wasnt functioning. I installed both rams individually on both slots, but the PC wouldnt POST. The PC works perfectly with the rams on slots B1 and B2.

I understand that this setup would reduce my ram from dual channel to single channel, but the performance drop isnt usually noticeable in conventional use. I still do want to fix it though if i plan to change to 4 slots of ram.

I have sourced the internet for days, but havent found any identical issues to mine for the fact that it used to be functioning for 1 year before this sudden error. Other people with the same error has it during the assembly.

I can list some of the error source i found online:
a) CPU wasnt installed properly into the back plate causing the memory controller for one channel to not work properly
b) Memory Compatibility issues fixed with the MemOK! switch

I doubt the reasons above apply to my case though as it was working for 1 year prior.

Can anyone help me?

Thanks in advance