I would think it has to be a short somewhere.
I am working on one with the same problem. It is driving me crazy!!!
Grounding the tristate doesn't keep it on. So it has to be hardware, somewhere.
Changed the PSU.
Changed the power/eject board
Changed all cables.
The thing is I have a multimeter, but it is a crappy cheap one. I may have to invest in a good one, to get some accurate tests.
I think it may be that fuse, I have seen you talk about in a couple threads.
But I can't find that one thread that had the pics in it.
Well, I guess I keep searching. /rant