one of my DIMMs (actually the best Dimm i have) is half dead for more than 5 months
actually the dimm CANNOT post in single channel (one dimm) in Dimm2
my other dimm boots no problem
so what i do is this (works every single time)
boot with the fully working dimm in dimm2, wait till it posts ok then power off the board
WITHOUT turning the PSU off i put the second (half dead dimm) on the dimm4 then i power on the board --> everything ok.
if a PSU power-off is involved i get cold boot and i have to do the above proc all over again...
hope it helps somehow