INVESTORS INCREASE STAKE IN FROST, SULLIVAN One of a pair of private investors in Frost and Sullivan Inc told the Securities and Exchange Commission he increased his stake in the firm by about two pct, to 15.4 pct. He is Theodore Cross, Princeton, N.J., editor of Business and Society Review. The other investor is Mason Slaine, Cos Cob, Mass, president of Dealers' Digest Inc. He holds a 3.3 pct stake. Cross told the SEC he bought the new shares at between 7.75 and 8.0 dlrs.