U.S. stocks slipped on Wall Street but were still near record highs at Monday's midday trading. Investors are looking ahead to Fed chair Janet Yellen's remarks over the next two days. On the economic front, existing home sales hit their lowest level in nine months, falling 4.9% in January. Big stock moves on this Merger Monday include a 13% swing to the upside for Valeant Pharmaceuticals (VRX). The company said it will buy Salix (SLXP) for about $10 billion in cash. 3M (MMM) is pretty much flat but Polypore International (PPO) is gaining big after agreeing to sell its Separations Media business to 3M for $1 billion.