Advance Auto Parts (AAP) surged in midday trading Wednesday after hedge fund Starboard Value disclosed a 3.7% ownership stake. Starboard's new position makes it the sixth-largest shareholder. Stocks rallied after a disappointing start to the week. However, the S&P 500 remains on track for its worst quarter since 2011. Further signs of a slowdown in China have rocked markets over the past three months. Comscore (SCOR) was sharply higher after agreeing to acquire audience measurement firm Rentrak (RENT) in an all-stock deal worth more than $700 million. Manufacturing activity in Chicago fell into contraction territory in September, its weakest reading in four months. The measure portends poorly for the ISM manufacturing report out Thursday.