ok, I had this same issue... computer would not go fully into sleep mode, the monitor would be blank (btw I was not getting check cable message on monitor so I know it was connected. I also swapped DVI cables, etc.)
the problem seemed to be with going to sleep because I had disabled an audio card that I was getting poor audio out of (Creative SB X-Fi. still working this issue). I enabled the audio card, and now the computer goes to sleep properly and resumes properly as well...
so, check you audio cards too, as I know most mother boards have a built in audio (Realtek) and many folks add another higher end card and disable the other...
good luck!