I think "Intermittent" is going to be a problem sorting this one out.
Everytime I go to someone who has an intermittent fault it never happens when I am there.
I have a friend who has the same problem as you.
If he opens and closes the DVD draw a few times it works. He has two optical drives a DVD Rewriter and a DVD Rom. Have tried a new Sony DVD Rom drive but (for some reason that actually stopped the computer booting up.) Tried a new Pioneer and that also stopped the computer booting. Put his old original DVD Rom drive back and the PC boots OK but still get this intermittent (can't read) problem. Fortunately, his DVD Rewriter doesn't appear to have any problems so he is only using that one at the moment. I have tried his Pioneer DVD Rom in my PC and it is working perfectly.