Despite his commitment to Third World buccaneering, Fidel Castro is desperate to reduce his military involvement and embarrassing losses in Angola by seeking a Communist partner, East Germany - so far with no success.

Castro certainly does not want to weaken the world's perception of Communist Cuba's desire to export Marxist revolution. Rather, he asked help from the East Germans because he badly needs a temporary reprieve from an increasingly impossible mission: stamping out the civil war waged in Angola by Lucas Savimbi's guerrillas against the Soviet-backed Marxist government of Agostinho Neto. The only reason Neto has survived is intervention by Castro's Africa corps.

In return for Castro's indispensible help in propping up the Kremlin's shaky man in Angola, the Soviet Union sends huge subsidies to prop up the shaky Cuban economy. Yet, in President Carter's State Department, this crucial service performed by Castro for the Soviet Union is deemed inconsequential. It is "no burning issue," Assistant Secretary of State Richard Moose recently told reporters.

What is "no burning issue" to Moose, top U.S. policymaker on Africa, is Moscow's indispensible tool to penetrate deeper into black Africa at Western expense. But the tool, Fidel Castro, is getting trapped in a political quagmire from which he can find no way to withdraw.

Instead of fulfilling his "pledge" last year to then Swedish Prime Minister Olaf Palme to withdraw his 13,000 to 15,000 Cuban troops at a 200-a-week rate, Castrol actually has been increasing his military commitment. During this part summer, five Cuban transport convoys are believed to have sailed to both military and civilian advisers.

There is little doubt here that this was a net reinforcement to bolster Neto's precarious situation following the barely aborted coup attempted against Neto last May. Cuban help was vital for Neto.

The troop reinforcements probably put Castro's Africa corps close to 20,000. Despite the increase, Castro's shrouded efforts to reduce his commitment are no longer doubted by Cuban experts here; nor is his growing concern about the domestic political effect of his Angolan escapade.

Castro's brother and power-sharer, Raul, is believed to have made a direct approach for help to East Germany during his visit there two months ago. One argument: The dispatch of tough, pro-Western Moroccan troops to protect pro-Western Zaire from a Soviet-financed invasion by mercenaries based in Angola poses a new threat to Castro's own troops in Angola. Forces from fraternal Communist countries - East Germany, for one - were accordingly needed to share the burden with Castro. So far, ther have been no takers in East GermanY.

Castro is showing signs of deep concern over Cuban casualties in Angola, and the political effect on 9 million Cubans at home.

The Cuban troops sent to defend Neto's worsening position in the Angolan civil wear are carefully picked from widely scattered provinces and towns to minimize the local effect of casualties.

At the same time, Castro is making an-extraordinary number of speeches in Cuba proclaiming nonexistent economic good times. In no fewer than nine speeches since early July, Castro's theme was economically upbeat - with repeated allusions to the low cost of Cuba's foreign adventures. The apparent objective: to deflect growing about Castro's immense investments in far-off places.

If Castro's Angolan adventure had been a brilliant success, his buccaneering thousands of miles from home might make glamorous contrast, to Cuba's economic misery. Instead, he has a costly stalemate in Angola and an ever-rising economic dependence on Moscow at home.

Soviet aid to Castro is running at a record peak of over $1.5 billion a year. That includes Soviet purchase of 2.5 million tons of sugar at 30 cents a pound - an exorbitant subsidy of more than 20 cents. Moscow also sells Cuba 9 million tons of crude oil and crude-oil products at about half the world cost.

The price exacted by these vast subsidies is the cost of Castro's domestic insurance policy: So long as Castro preserves Neto in Angola, thus protecting the extension of Soviet influence into the strategic heart of southern Africa, he can count on Soviet largesse. The wonder is that all this of so little interest to Carter's policymakers.