Capitol Hill splits: Rep. Robert Davis and his wife of 11 years, Marty, have separated. A former broadcast journalist, Marty Davis is outspoken and has been involved in a couple of incidents that caused her husband political difficulties. One was two years ago when a photograph of her in a sexy exercise leotard made the newspapers. She said then it was her way of showing that one couldn't generalize about congressional wives -- that they all weren't frumpy, blue-haired ladies. More recently, she was the subject of several critical stories in newspapers in her husband's district in Michigan's Upper Peninsula. She had written a letter to the editor, criticizing opponents of congressional pay raise legislation and pointing out how difficult it was to live on a representative's salary. Davis would not comment yesterday on the trial separation. His wife would only say "there were unresolved political differences." The couple has a daughter, Alexandra, 4 ...

Sen. Steve Symms revealed Wednesday that he and his wife of 27 years, Frances, were living separately. His wife would not comment on the separation, but Symms, who moved from the family home in Alexandria, said, "The only thing I'm saying is that I'm separated from my wife. I'm like a lot of other people. I'm having a few domestic problems ... There isn't anything I can say that will ever help." The couple has three adult children and one at home. Out and About Not many people realized it, but a third-grade graduation ceremony delayed the testimony of Iranian-born businessman Albert A. Hakim before the House and Senate committees investigating the Iran-contra affair yesterday morning. It seems that Hakim's attorney Richard Janis, with a surprising sense of family priorities for a Washington power broker, was able to get a delay in the testimony so Janis could see his son Taylor graduate from the third grade at Beauvoir, the National Cathedral elementary school. Everything was back to normal later in the day when Hakim, with Janis at his side, returned to the hearings to continue his testimony ...

The big three, tenor Placido Domingo, Gian Carlo Menotti and Rafael Fru hbeck de Burgos, who brought the Washington Opera's world premiere of "Goya" to the Kennedy Center last season, will be back here and working together to open the Washington Opera's 1988-89 season. Domingo will sing the role of Mario Cavaradossi in a new production of Puccini's "Tosca," with Menotti directing and Fru hbeck conducting. The opera will open Nov. 5 ...

Former president Richard M. Nixon sat in New York's posh Le Cirque restaurant yesterday for a long lunch with his former White House counsel, Leonard Garment, and superlawyer Edward Bennett Williams. In the transcripts of the Watergate tapes, Nixon expressed little love for Williams, who was then counsel for the Democratic National Committee, and was quoted as threatening to "fix the SOB." Yesterday's lunch reportedly lasted about three hours. Maybe Williams and Nixon were ironing out old differences ...

Sen. Howard Metzenbaum celebrated his 70th birthday yesterday with a surprise gift from his staff. At a Capitol Hill party, his daughter Shelley, who lives in Boston, showed up with her 2-week-old daughter Leora. It was the first time the senator and his wife Shirley had seen Leora, who is their fifth grandchild ...

Bad-tempered actor Sean Penn is being sued by the movie extra he allegedly pounded about for taking a photograph of him during the shooting of the movie "Colors." The suit filed by Jeffrey Klein in Los Angeles County Superior Court also alleges that Orion Pictures and other companies involved in producing the film were negligent for subjecting employes to Penn's reputed temper. As Klein's attorney put it, "It's sort of like having a dangerous dog where you work, and the dog's been known to bite before" ...