Costco's stock has stumbled even as its warehouse business continues to thrive.Costco stock has slipped back after a long stretch of outperformance. As of this writing, shares of Costco Wholesale (COST +0.30%) are trading below $900 again, putting the shares in the red for 2025.The wholesale club remains one of the strongest retailers in the market. Costco's membership model and private-label brands continue to pull in steady traffic and high renewal rates. The problem for investors is not the business -- i ...