iPhone 4 forgetting calendar preferences

Recently, my iPhone 4 refuses to remember which calendars I want it to display and which ones I don't. I have a calendar called "invisible reminders" which alert me to all manner of things daily, but I don't want to see them in the calendar, they are just there to create alerts. When I uncheck that calendar in the list, it stays that way until an hour later or a day later, in which case it has been re-enabled.

Any ideas?