road bike, horizontal dropouts, mavic wheelset, with mavic QR. I got this excerpt from a site:
"With horizontal dropouts, it is possible to mis-align the wheel in the frame if it is installed carelessly. The axle nuts or quick-release must be tightened quite securely, or the chain tension may pull the axle askew."
This is what's happening, I'm almost certain. It is possible then that I need to tighten the quick release very tightly? I remember reading somewhere that excessive tension could cause damage.
EDIT: when I secure the wheel, I can center it just right, but as soon as I get going, I believe the chain tension is causing the wheel to twist. The problem does go away when the QR is very tight - is this necessary?