It's not unusual for a script to run for some time, and then get quota errors all of a sudden. I have no idea why. I don't know if Google, kinda, "let's it go for a while", and then "reigns you in". Or maybe they are just slow at figuring out that you're over quota. All I know is, that I've heard the same complaint that you are experiencing quite a few times. So, "Why does it happen?" Who knows? But, about all you can do is optimize your code, and make sure that you are throttling service calls in situations where you have service calls in a loop. So, do you have service calls inside of a loop? The classic example is setting Sheet cell values individually, one per loop.