Big-cap leaders played a stalwart role through the past few weeks. For the most part, they held steady as the market came under pressure and as major indexes remained below resistance levels. But in the past week, a handful of Big Cap 20 leaders have begun to wobble. The weakness goes hand-in-hand with the Dow's fourth week of frustrated efforts to crack the 10,500 level. Diminishing volume among big-cap leaders suggest institutional investors