Calendar events from the past randomly disappearing

I have my iPhone, iCal, and MobileMe set to sync calendar and contacts through the air.

Occasionally, random past events in the calendar will disappear - this also happened to me sometimes before I used MobileMe, while I was syncing to iTunes manually.

I don't have any of the options for deleting past events enabled in iCal. Also, I can't seem to detect a pattern as to which events get deleted - there isn't a "cutoff date" beyond which events disappear, some of them stay and some of them don't.

This is particularly annoying as I use my Calendar as a historical record of days worked, meetings, etc, and I don't feel like I can trust it.

Which settings should I use so that nothing ever gets automatically deleted from the calendars on my iPhone, my Mac, and MobileMe, and the databases stay as a complete archive?

Thanks,

Derth Adams