04/21/2004 at 01:00 AM EDT
Whitney Houston's erratic behavior of late has been the cause of concern for her friends and relatives, reports this week's PEOPLE. Drugs are being suspected, despite denials issued by Houston and her publicist. "Dionne (Warwick) and Natalie (Cole) were going to talk to Whitney about her problem," a source close to the pop diva, 36, tells the magazine. "Natalie is really the voice of authority on this because of what she's been through with drugs. But they didn't talk to her." Since the beginning of this year, Houston has made headlines: she was caught at a Hawaiian airport with marijuana in her bag; she missed a salute to her mentor Clive Davis at the Rock & Roll Hall of Fame; she was replaced on the Oscars and she gave a disjointed interview to a magazine. On Monday she is expected to perform at Arista Records' 25th anniversary party. All eyes will be on her.