Vanishing iCal events

After discovering that trying to be too clever with multiple syncing was making iCal run impossibly slowly, I am experimenting with simplicity. I have a single iCal calendar, which I back up to our TimeMachine and sync with a single Google calendar. Nice, fast functioning now but I am regularly faced with the horror that events entries which have yet to happen are mysteriously vanishing! Any help with this will be gratefully received; my life is thinly veiled chaos as it is and I rely heavily on a calendar to keep me vaguely organised...