I had this on a nightmare G5 refurb from hell I had. Apparently this particular issue is something they see often... I took it in and had them fix it (it was in for other things, too). I tried messing with it myself several times and managed to make it better but not fix it. If it's possible, take it in. If not, I'd look at trying to find a new set of standoffs -- you need just a couple extra mm of height at the back end of the drive and the whole thing should be aimed better.