iCal occasionally can't log into my CalDAV GCal account

I have one Google account that has a calendar with a bunch of sub calendars, and I have iCal and my iPhone syncing with them via CalDAV. Everything was working just fine, until a few days ago when iCal said it couldn't log into my GCal account to sync with it, even though I had saved the password and nothing changed. It pops up a login window for every one of my sub calendars once in a while, which is incredibly annoying. I have it set to sync every 5 minutes, but it doesn't pop up the message every 5 min, which means sometimes its working and sometimes its not. All I have to do is enter my password and it goes ahead and syncs, but its incredibly annoying and should not be happening. Any ideas about what's going on?