Average Cost discrepancy

We are a distribution (importer) company using Advanced Inventory and we use FIFO accounting.  When we receive inventory, we have duty, freight and many other misc landed cost items we enter with the receipt.  Initially, the average cost seems to be in line with what it should be based on the first cost, and the additional landed costs.  However, once the inventory gets down to the last few pieces, the average cost seems to get way out of whack.  Maybe our average cost starts out at $5.00, it could end up being $117.00 later when we only have 2 pieces