Senior Reagan administration policymakers had serious doubts until recently about the veracity of Maj. Roger Miranda Bengoechea, a defector from the Nicaraguan Defense Ministry who described plans for a significant Soviet military buildup in Nicaragua, sources said yesterday.

The sources said Miranda changed his story several times and had difficulty passing a lie-detector test administered by U.S. intelligence agencies. "A lot of his information had to be double- and triple-checked," said one informed source. "Some of it wasn't adding up."

These doubts apparently were one reason why public disclosure of Miranda's information was held up until after last week's superpower summit, according to informed administration sources. Other officials said Miranda's story was withheld because of a judgment by State Department officials that it would be overwhelmed by the summit.

The sources said they have renewed confidence in Miranda's information because of a speech made by Nicaraguan Defense Minister Humberto Ortega on Saturday in which he confirmed key parts of Miranda's account of the planned buildup. "I don't think {Miranda's} credibility is in question" given Ortega's address, said one official.

Ortega's speech on the planned Soviet buildup, which he said would include enough arms for 600,000 full- and part-time soldiers by the mid-1990s, drew sharp criticism yesterday from House Speaker Jim Wright (D-Tex.) and others on Capitol Hill.

Wright said that such a buildup would be "preposterous." He described Ortega's speech as "outrageous and counterproductive" to the regional peace plan signed by Nicaragua and four other Central American nations. Wright said he had communicated through an intermediary to Sandinista Foreign Minister Miguel d'Escoto that Ortega's comments were "intemperate and irrational." An aide said the message was sent Sunday.

Also on Sunday, Ortega's brother, Nicaraguan President Daniel Ortega, described the plans for a buildup as merely a military "proposal" that had not been accepted by the Nicaraguan regime.

Wright, a leading advocate of the regional peace plan, said yesterday that Humberto Ortega's speech could make it easier for the Reagan administration to win congressional approval for new nonlethal aid for the Nicaraguan rebels, the contras. Sen. Pete V. Domenici (R-N.M.) said Sandinista confirmation of the planned military expansion will make it "very easy" for contra aid supporters to succeed.

The Senate early Saturday approved a continuing resolution that includes an additional $9 million in nonlethal aid for the contras, plus up to $6 million more to cover transportation costs. The money is intended to support the contras through February. The House version includes no such aid. The bills are to be negotiated this week in conference.

Miranda is being used in the administration's bid for continued contra aid. He met yesterday on Capitol Hill for 2 1/2 hours with a bipartisan group of 25 moderate lawmakers. He also continued to give interviews to the news media. The interviews are arranged and monitored by aides to Assistant Secretary of State Elliott Abrams, a leading proponent of continued contra assistance. The first interviews under this arrangement were given last Thursday. State Department officials had set up the interviews on the condition that the information not be published until Monday. The Washington Post published it Sunday after Humberto Ortega made his speech confirming plans for the buildup.

An administration official said there had been a debate over whether to air Miranda's charges before the summit, with some officials arguing that an effort should be made to publicize the disclosure before the arrival of Soviet leader Mikhail Gorbachev. However, this official said others urged caution because of the backfire of previous efforts to exploit a defector.

White House spokesman Marlin Fitzwater said yesterday that the administration lacked "confirmatory evidence" of Miranda's statements last week, when Gorbachev was in Washington, but that Humberto Ortega had confirmed "most of it." He added, "That accounts for most of the timing."

White House officials said President Reagan did not specifically bring up the disclosures from Miranda during his summit meeting with Gorbachev, but the president did ask Gorbachev about other evidence obtained separately from Miranda showing a continued Soviet buildup in Nicaragua.

Gorbachev hinted during the summit that the Soviets would be willing to suspend major arms deliveries to the Sandinistas in the context of the regional peace plan and termination of U.S. support for the contras. Gorbachev was apparently not asked to clarify this offer. Yesterday, White House officials said the Soviet Union will be asked today for a clarification of Gorbachev's remark.

Staff writers David B. Ottaway and Tom Kenworthy contributed to this report.