For reasons that have many observers here baffled, Polish authorities again have lashed out at the United States, pouring new vitriol on relations that were just beginning to show signs of improvement.

The fresh wave of attacks has been led by communist party and government head Gen. Wojciech Jaruzelski, who last week accused the Reagan administration of reneging on pledges to restore Poland's fishing privileges in U.S. waters, landing rights for the Polish airline and exchange programs for scientists.

Elaborating on the allegations, government spokesman Jerzy Urban told reporters: "The U.S. government's declarations of a desire to improve relations, at least in some areas, are not followed by real decisions, and the facts are often at odds with the words."

Repeating the complaint, Stefan Olszowski, the Polish foreign minister, said U.S. policy toward Poland was illusory and "a game of appearances." In a speech to parliament, he criticized Washington for rejecting a Polish appeal for talks on U.S.-Polish issues and complained that Washington recently had given Poland an "ultimatum" to accept a new U.S. ambassador to Warsaw or face a further deterioration in relations. The ambassadorial post has remained vacant since February 1983.

Poland's verbal assaults followed the much publicized expulsion of a U.S. military attache Feb. 25 on spying charges.

The episode drew countercharges from the Reagan administration that Polish police had mistreated the attache, Col. Frederick Myer, and his wife.

It prompted the United States to expel a Polish military attache from Washington and to postpone indefinitely a planned resumption of scientific exchanges.

The matter has left Washington once again irritated with the Jaruzelski government.

Why the Poles chose to oust Myer, making a big issue out of an incident that governments normally handle discreetly, perplexed some western and Polish specialists.

One theory is that the police action and recent angry speeches serve to accommodate orthodox elements in the leadership and security services who resented the trial of four secret police officers, convicted in February in the murder of pro-Solidarity priest Jerzy Popieluszko.

Others see the foreign policy moves as part of a general pattern of panicky behavior exhibited lately by the Jaruzelski government. Worried about worker unrest over recently announced price increases, communist authorities have stepped up the arrest of opposition activists and aggravated tensions with the Roman Catholic Church.

Although these actions preceded last week's change of leadership in the Kremlin, some western diplomats suspect that Warsaw is also nervous about what new demands Mikhail Gorbachev, the Soviet leader, may make on Warsaw.

To the astonishment of U.S. officials, the Polish Foreign Ministry summoned the U.S. charge d'affaires a week after the expulsion of the military attache to deliver a plea for new financial credits. Poland is facing interest payments of $2.7 billion this year on its western debt.

Possibly seeking to pull back from last week's anti-American diatribes, Urban told reporters yesterday that Poland is "not attacking the United States" but "simply replying to attacks on Poland" by such officials as the U.S. representative to the United Nations Human Rights Committee in Geneva.

Affirming Poland's interest in economic cooperation with the United States, Urban said: "We expect the United States to stop its attacks on us, then arguments from our side will cease immediately since they will no longer be necessary."

U.S. officials deny any bad faith on their part in trying to restore suspended agreements with Poland. The State Department asked Warsaw in February for an answer within 30 days to the renomination of a career foreign service officer, John Scanlan, to be ambassador to Poland.

The deadline has now expired. The U.S. position on Poland's proposal for talks, has been that they would only be advisable after a U.S. ambassador has been installed.