A sudden and sharp downturn in semiconductor shares led U.S. share prices sharply lower mid-afternoon Monday, erasing early gains from a better-than-expected national manufacturing report. In early afternoon, the Philadelphia Semiconductor Index was down 44.94, or 8.2 percent, at 500.10, a fourth month low, hitting many chip stocks hard. The Nasdaq composite finished down 3.11%, 57.29 points, to 1782.97, a new low for the year and back at late 1998 levels.