ALTHOUGH the cold war has ended across most of the world, it still drags on for Zhen Lijun and thousands of other residents of fishing villages on the Taiwan Strait.Nationalist troops from Taiwan recently killed Mrs. Zhen's cousin with mortar fire as he sailed his fishing boat near Quemoy, an island off the communist mainland held by Taiwan. (See story, below.) The young mainland fisherman was one of at least 20 similar casualties this year in China's stubborn conflict between capitalism and communism, according to mainland statistics. Fishermen tell a story of strife between mainland Communists and Taiwan Nationalists that differs from the steady rapprochement widely reported in the past several months. Indirect trade in 1991 is likely to far exceed last year's record of $4 billion and Taiwan companies have invested an estimated $750 million to $2 billion in the mainland since 1987. Also, Taiwanese officials in April declared an end to the war with Beijing and has acceded to contacts that are all but official. It has opened a Beijing office and last month allowed mainland journalists and Red Cross officials to visit for the first time. But the rival governments show no sign of dismantling the ideological underpinnings of their dispute. Both Taiwan and the mainland consider themselves the political center of China and have tried to convert the other to their form of government and economic style for the past four decades. Tension across the strait will probably intensify because of the profound changes in the Soviet Union, say scholars and officials on Taiwan and the mainland. The breakaway of Soviet republics from central control has spurred Taiwanese opposed to what Nationalists and Communists call the eternal bond of Taiwan to China. The Democratic Progressive Party, Taiwan's leading opposition group, has recently stepped up its call for Taiwanese independence. The fall of Soviet communism has had other effects. It "simply vindicates what we have been saying for 40 years, that communism is not suitable for China," said Ma Ying-jeou, deputy chairman of the Mainland Affairs Council, in a telephone interview from Taipei, Taiwan's capital. The council is Taipei's leading body for handling relations with Beijing. China's leadership has condemned Taiwan's independence movement and Taipei's efforts to promote a democratic, free-market system on the mainland. The changes in Moscow will make Beijing feel more beleaguered and combative, Mr. Ma says. Still, the Nationalists and Communists must engage each other as each tries to promote reunification on its own terms. For example, if Beijing flaunts its military superiority, it will antagonize Taiwanese and compel many of them to embrace the extremes of either hard-line Nationalists or independence activists. "The mainland must court Taiwan; it will not work to try and make this a forced marriage," says Chen Kongli, director of the state-sponsored Taiwan Research Institute at Xiamen University. Similarly, Taiwan fears that if it pushes trade and investment too far, it will grow dependent on the mainland. Taiwan's investment in Xiamen, a port in Fujian province across from Taiwan, illustrates how weapons in the close, fraternal struggle can easily cut both ways. Dynamic businesses from Taiwan have leapt into the role as the most potent champions for capitalism on the mainland, say the scholars and officials. Taiwanese managers are attracted by lax environmental regulations, a cost of labor one-tenth of that on Taiwan, and the sentimental appeal of investing in their "homeland." In Xiamen, the executives find the mainland's siren song of opportunity and nationalism especially irresistible because of a common dialect and shared customs, officials and scholars say. Taiwanese companies had agreed to invest a total of $1 billion in 410 projects in Xiamen by the end of last year, according to the New China News Agency. But the Nationalists are worried by the ballooning investment and a jump in trade of 43 percent in the first nine months of this year. Taipei bars direct trade and plans by year's end to enact laws aimed at ensuring that indirect trade (currently 4 percent of total imports and exports) does not grow too fast. Taipei reckons it will be dangerously dependent on Beijing if 10 percent or more of its total trade involves the mainland. In particular, Taipei fears Beijing will try to blackmail it by threatening to cut off shipments of raw materials. And Taiwanese executives could act as advocates for the political interests of Beijing because of their extensive investment on the mainland. For its part, Beijing also views Taiwanese executives as potential threats and allies, for different reasons. In an effort to bind Taiwan to the mainland, Beijing offers companies a tax holiday of up to five years and other preferential treatment. But it tries to ensure Taiwan's money remains apolitical. "China will continue to limit cooperation with Taiwan to science, technology, and business management," Governor Jia Qinglin of Fujian province told the Monitor. "We will bar Taiwan compatriots from engaging in politics or anything political." The mainland in recent months has stepped up controls over contact with Taiwan. Its customs handling of pamphlets and videos from the island has been especially tight since July, Ma says. Also, administrators at Xiamen University forbid students from tuning in to Taiwan radio and television, according to several undergraduates. Nevertheless, through its radio, TV, and thousands of factory jobs in Xiamen, Taiwan has succeeded in coloring the outlook of citizens here, says Dr. Chen. Along the port's narrow streets, neon signs over Taiwan-owned karaoke nightclubs flash in syncopation with Taiwanese pop music blaring from shops. Xiamen devours Taiwan's message. Residents ranging from students to factory workers speak wide-eyed about the TV game show "One Hundred Battles, One Hundred Victories," the soap opera "Stars Know My Heart," and the slick ads in between. "Tell Taiwan to build more broadcast towers," a taxi driver told a visiting journalist. "Their TV is freedom in a box."