Re: ASUS M4A87TD Won't boot with two sticks of RAM. Works with one.
It could be anything at this point, you'll just have to play around with the voltages, timing, speed... and see if anything works.
Like you said, could also be bad ram slot on the mobo.
I don't know if this is possible or not, but use just 1 stick of ram and test out all the ram slots on the mobo to see which slots work and which don't work.