Market breadth is the key factor Nathan Peterson is watching in the S&P 500 (SPX). He explains why "there's some healing that needs to be done" in sectors beyond tech. Also mind the volatility he sees hitting markets at Wednesday's close due to the combination of Micron (MU) earnings and quarter-end rebalancing. Nate looks ahead to October and tentative headwinds he sees weighing on Russell 2000 (RUT) technicals.