A three-month-old slide in shares of Silicon Laboratories ( SLAB) worsened Tuesday after the specialty chipmaker issued third-quarter guidance that stunned Wall Street. The Austin, Texas, company reported a strong second quarter with 11% sequential revenue growth and a 5-cent earnings beat, but said third-quarter revenue would be $120 million to $123.6 million, well below the Thomson First Call consensus of $129.5 million. Silicon Labs cited a sequential decrease in revenue from its mobile handset business. The stock, which cost $52.75 on June 1 and $59 on April 8, closed at $39.80 Monday before the news was released. Tuesday morning, the shares were down $6.01, or 15%, to $33.79. In the second quarter ended June 30, Silicon Laboratories earned $22.8 million, or 41 cents a share, compared to earnings of $10.9 million, or 21 cents a share, last year. Revenue was $126.1 million in the 2004 quarter compared to $69.1 million last year. Excluding stock-based compensation chares, Silicon Laboratories earned 43 cents a share in the second quarter, compared to the Thomson First Call estimate of 38 cents a share.