I second ScottCommutes. Yes, it is probably time for a new tire. But, not finding the cause means simply changing the tire out does not guarantee the flats will stop happening. What if they are from a rim burr, slippage of the rim strip, something in the rim strip ... Plus finding the cause educates you a little more on perhaps likely causes of flats in your area. Knowledge doesn't prevent all flats. But sometimes lack of can lead to many.