Sounds like a good plan to me. If you intend to sell the car, first try some of the commercial stop-leak products and see if that helps. If so, you saved a LOT of time and money, and will be able to sell the car leak-free (well, leak-reduced anyway). If you keep it, diagnose that "slipping" before you make the decision to reseal or replace. If it's NOT slipping, just reseal it.