Asus A8N-E Motherboard Memory Slot Problem (Please Help)

Hi I'm not a tech person, so I have a query:

My PC has been working fine for 4 months. But then suddenly it wouldn't start up, it will not even go to post, black screen and a long beep then 3 secs then another long beep, endlessly looping. I thought it must be the memory.

I have 2 x 512MB PC3200 400mhz in the blue slots & 2 x 512MB PC3200 400mhz in the black slots, all slots are full making a total of 2GB all the same make & speed etc... identical.

I took out the memory from A2 & B2 slots and it works fine, boots up perfectly with 1024MB. If I move the 2 sticks to A2 & B2 slots then it will not boot up at all, same beeps.

If I move the DRAM Sticks to the A1 & B1 slots, fine again. If I leave those in there and add a third stick to the A2 slot: it boots up but only recognises 512MB DRAM installed.

If I take that out and move it to slot B2 the PC won't start, same beeping.

So should I conclude that the B2 slot farthest from the CPU is faulty & fails to register the DRAM?

I don't think it's the actual DDR as any combination in slots A1 & B1 work fine.

I also tested each individual DRAM chip in Slot B1 (says in user manual, if using 1 DRAM Module insert in B1) PC started perfectly with each 512MB Memory chip and I then used Check It Daignostics to run all usual Memory Tests, all Passed, & thus obviously rules out memory problems, must be MB, right?

So has the motherboard failed? Should I return it to the vendor?

(posted under motherboards too)