Desktop Icon Positions

My Desktop icons seem to be deciding for themselves where they should appear. I have tried several times to keep the icons arranged the way I like - "By Type - Directories First". The arrangement lasts through maybe three logins, then I find the icons rearranged again. If it matters, I'm using Trinity 3.5.12 with Debian Squeeze. I have checked the file ~/.trinity/share/apps/kdesktop/IconPositions and nothing obvious is amiss, though I can't be certain I'd recognize a problem there. I'd be grateful for a clue. Thanks John