There's a length ramble about this in
my blog, but the short-n-sweet version is this: Following a few months in which the goddamn thing would move off the D0 every few weeks, I gave up and ordered a X2Pro to replace my Matrix. Then, some mates of mine announce that they want to swap my V1.0 chipped Xbox for a new unchipped one that I could then fit my X2Pro to. I like this, as I'll get a better optical drive (Thomson at the moment) and get rid of the noisy 40mm fan on the GPU, but now I have to fit the bloody Matrix again for them. The guy who's going to solder the X2Pro for me is out of town so soldering the Matrix is out unless I'm going to put off Apex for a week.
By now, with all the dicking around it's had, the screw post below the Matrix is thoroughly reamed and stands no chance of holding anything to anything. So, we took extreme measures. 3.5mm drill bit, drilled down the post until it came out the other side; 3mm bolt stolen from a fan mouting in my PC, through the chip, the board, the bottom of the case and out the other side. Dremel the overlong bolt down, fit a nut and washer, and Robert's your mother's brother. Took about ten minutes, hasn't disfigured my Xbox much, and there is now no way that chip is going to move.

So, if anyone else is having similar problems, I say you should bolt it. Worked for me. Possibly this is a well-known trick; I've not been keeping up to date with this forum, I must admit. In that case, please accept my apologies for wasting your time!