NY- Shock jock Howard Stern got the scare of his life when a crazed fan spit in his girlfriend's face and threatened both of them on an upper West Side street, sources said yesterday.

But a frightened Stern quickly turned crimefighter, helping cops nab the screaming, arm-waving lunatic, the sources said.

Stern, who is regularly approached by friendly fans on the street, was particularly shaken by the "deranged" man's angry outbursts, a source close to the radio host said.

His nerves, however, were calmed by a dozen or so cops who sped to the scene.

"It was very upsetting and traumatic, but he was very pleased with the professionalism and responsiveness of the police," the source said.

The story - which the privacy-loving shock jock may never talk about on his show - unfolded shortly after 8 p.m. Saturday as Stern and leggy girlfriend Beth Ostrosky were walking on Columbus Ave. at W. 62nd St. after dinner.

Wide-eyed fan Gregory Forbes recognized Stern and confronted the longhaired satellite radio gabber, screaming, "Howard!" Forbes, 50, allegedly screamed and waved his arms, threatening Stern and Ostrosky - getting right in their faces and making them fear for their lives, the sources said.

When the couple walked away and jumped into a cab, Forbes chased after them with his fists raised and spit through the taxi's open window into Ostrosky's face, a court complaint charges.

Stern managed to call cops - but by the time they arrived, Forbes was gone, the sources said.

Undaunted, Stern hopped into an unmarked police car with anti-crime cops and helped them scour the area for the weirdo, sources said.

Blocks away, two detectives spotted a man fitting Forbes' description about 8:50 p.m. Stern, from the safety of the unmarked police car, identified Forbes as the suspect, sources said.

Forbes, who lives in a boardinghouse in Hamilton Heights, was hauled to the 20th Precinct stationhouse on W. 82nd St., the sources said. He was arraigned last night in Manhattan Criminal Court on charges of third-degree menacing and second-degree harassment and held on $500 bail.

A representative for Stern, who lives with Ostrosky in Manhattan, could not immediately be reached for comment.

The weekend menacing incident was not the first time violent fans have threatened Stern. Ten years ago this month, a shotgun-packing man was busted in midtown after allegedly running after Stern's limousine as it pulled into a studio loading dock. In 1999, a man who once pointed a toy gun at Ronald Reagan in the 1970s was sentenced to 21/2 years in prison for sending letters to Stern threatening his life.

The Saturday night confrontation was the latest agita for Stern, who recently complained that not enough of his fans have followed him since he left free radio for pay radio in January. "I want to say to my audience . . . 'You haven't come with me yet? How dare you? We're up to wild, crazy stuff, the show has never sounded better,' " Stern told Entertainment Weekly.

He is also battling his old bosses, who claim in a lawsuit that he improperly plugged his new Sirius program on CBS' airwaves. Stern has denounced the lawsuit as frivolous - even going on CBS' "Late Show With David Letterman" to make his case.