In low light always use a tripod and, preferably, a remote release; and don't rely on what your camera tells you to be correct exposure. If in doubt, bracket.
Try bumping up your ISO and, I am guessing you are using a tripod. You might also try doing some manual focusing as well.
I shot this one at ISO400,f/8 in Apeture Priority. Granted it is a sunrise and, not a sunset. Remember at sunset the air is more unstable because the air is cooling too.
http://Joves.smugmug.com/photos/280137141_ZY5sW-M.jpg