Ivan F. Boesky's sentencing has been delayed a second time. Edward Schwartz, a spokesman for the U.S. attorney's office in Manhattan, said that the date has been changed to Dec. 18 from Oct. 9. Boesky originally had been scheduled for sentencing Aug. 21. Schwartz said he did not know the reason for the second delay, and Boesky's lawyer, Harvey L. Pitt, did not return phone calls seeking comment. But earlier this month, Pitt said sentencing might be delayed again for his client, who has been cooperating with federal investigators. Formerly one of the best known and wealthiest speculators, Boesky paid the Securities and Exchange Commission a record $100-million penalty last November to settle charges of insider trading. In April, Boesky pleaded guilty to one criminal charge of conspiracy to lie to the SEC.