Defense Department procurement chief John A. Betti, buffeted by criticism over his management of expensive new weapons programs and threatened with plans to reduce his job responsibilities, resigned yesterday.

Betti, who was appointed by Defense Secretary Richard B. Cheney amid promises that he would invest greater authority in the job and use the position to help clean up the Pentagon's beleaguered procurement system, became the third consecutive acquisitions chief to leave the job in bitterness and frustration.

Betti, who held the No. 3 job in the Defense Department for 16 months, was criticized earlier this week by the Pentagon inspector general for his failure to vigorously pursue reported problems with the Navy's new A-12 bomber program.

But Pentagon officials and associates of Betti say the issues that led to his resignation went far beyond the A-12 investigation and point to major problems endemic to the Defense Department's massive purchasing system and the position created to oversee it.

"It's an impossible job," a senior Pentagon official said. "So many people are stirring so many pots, from the deputy secretary of defense to Congress. How do you get hold of the job if people keep sniping at you and reorganizing your job under your feet?"

In a statement yesterday, Betti said he believes "that substantive, lasting improvements in defense acquisition will not occur overnight," adding, "Rather, it will be a long, slow and arduous process requiring patience and constancy of purpose."

Betti made no references to specific problems in his resignation letter.

"{I}t has been my intention to return to the private sector after making my contribution in federal service," Betti wrote. "By making my resignation effective before the end of the year, I will preserve some options under the ethics laws in connection with my future activities."

Ethics rules that go into effect next year are designed to slow the traffic between government and industry and curb potential conflicts.

Pentagon officials and Betti's colleagues said his decision to resign, effective Dec. 31, was based on a combination of criticisms from his superiors and frustrations on his own part.

Several officials said Deputy Defense Secretary Donald J. Atwood, Betti's immediate boss, has become increasingly displeased with Betti's performance. Officials said Atwood's most vocal criticism began after Betti's poor presentation last July before a House committee investigating the Northrop Corp., prime contractor for the B-2 bomber, the Pentagon's most expensive weapon system.

In recent weeks, David S. Addington, special assistant to Cheney, issued a memo recommending the creation of a job to oversee Defense Department research and development, now a key part of Betti's job.

"He disagreed on how his job ought to be redefined," one Defense Department official said. "He may well have viewed that as taking away some of his responsibilities."

But other Pentagon officials said Betti had isolated himself from his staff, creating major frustrations in the acquisition office because of a persistent lack of communications.

The acquisition chiefs who preceded Betti left their jobs complaining that they had not been vested with enough authority to help improve the Pentagon's troubled weapons buying program.

Richard P. Godwin, the first appointee to the congressionally created job, said he resigned in 1987 because defense officials ignored most of his recommendations for improving the procurement process and "the institution was not prepared to change the status quo."

His successor, Robert B. Costello, who left office after the Bush administration declined to keep him on the job, also expressed frustration over the difficulty of attempting to oversee the massive purchasing system.

The Bush administration offered the job to about 20 people before Betti, then a Ford Motor executive, agreed to take the position in 1989.

"For the third time in four years, the $100 billion Pentagon acquisition pipeline is without a leader," House Armed Services Committee Chairman Les Aspin (D-Wis.) said. "The position was created to prod the system to buy weapons that work on time and at cost. Betti's resignation shows how resistant the Pentagon is to change."

Betti's resignation, coming two days after the department's inspector general "shot him through the heart," an official said, focused new attention on the scandal involving the Navy's A-12.

Inspector General Susan Crawford told the House Armed Services Committee Monday that the government is investigating potential criminal wrongdoing in connection with cost overruns and schedule delays in the aircraft program. Betti was not linked to allegations of criminal wrongdoing, but Crawford criticized him for failing to adequately oversee the program and respond more aggressively to the problems.

"When abuses take place in procurement, heads should roll," Rep. John Conyers Jr. (D-Mich.), chairman of the Government Operations Committee, said yesterday. But "what happened to Undersecretary Betti doesn't address the underlying flaws in . . . procurements like the A-12."