Like other young couples aboard the interest rate roller coaster, Vernon and Leann Grapes spent the better part of the past five years looking for a house to buy and never quite succeeded.

After their marriage in 1978, the couple lived in two mobile homes, her parents' place and a rented house, all in western Maryland. In November 1981, they moved to a $360-a-month apartment in Laurel. Interest rates put even a no-frills, $50,000 home out of their reach.

Today, though, thanks to a Montgomery County program designed specifically for moderate-income, first-time home buyers, the Grapeses are owners of a $46,900, three-bedroom town house near the Rte. 29 corridor in Silver Spring. Over the past four years, they and 3,500 other families were helped by the Montgomery County Housing Opportunities Commission's offering of mortgages at up to three points below the going interest rate.

"Without the HOC program, we wouldn't have a house today," said Vernon Grapes, 25, a senior technician at an electronics testing firm in Falls Church. "Mongtomery County was on the ball with this one."

"The bonds and the lower mortgages get people into the ownership cycle, the American dream," declared Bernard L. Tetreault, HOC's executive director and president of the new 70-member Association of Local Housing Finance Agencies.

"The program has done so much for so many in the county," Tetreault said. "It's a viable mechanism to give people the opportunity to buy housing who otherwise wouldn't have the chance."

In Montgomery, a generally affluent county, the special mortgages have drawn the praise of developers and home builders, who, despite an unpredictable market, are virtually assured some units will sell if set aside for the HOC program. The mortgages are most popular among prospective home buyers, who flooded HOC with hundreds of telephone inquiries after a bond sale to finance the program was announced last week.

Three hundred people a day are calling the HOC's mortgage hot line (565-HOME) to leave their names and addresses with the commission, HOC spokeswoman Joyce Siegel said. HOC will mail callers information about the program, including details about income requirements and the 26 new home subdivisions participating in the program.

"We're tailored for the middle-income folks," Siegel said, adding that the first 20 applicants for the latest program include a painter, a private detective and a single parent.

HOC is a public corporation formed in 1966 as a traditional housing authority. It won state approval in 1974 to issue tax exempt bonds to finance the special mortgages. The interest on the Grapeses' mortgage, for example, which was approved after the sale of bonds earlier this year, is 11 1/4 percent. Last week, HOC announced the sale of $32 million more in bonds to finance 500 mortgages at rates of 10.87 percent, or about three points lower than the regional rate.

While the bonds have been the key to the success of the special mortgages--HOC has sold $300 million worth in nine years--they now also threaten to kill the program this year, HOC officials say.

Because interest on the bonds is exempt from federal and state taxes, governments are losing millions a year in revenues, say Reagan administration officials and other opponents of the program. A newly formed association of housing finance agencies is fighting the administration's proposed sunset legislation to end bonding programs in December.

For the Grapeses, the HOC program means a start to building equity, Vernon Grapes says. At the time he saw a television announcement about the HOC mortgages, the Grapeses were paying $400 a month in rent, he said. "We were just on the verge of being able to buy--and not being able to buy." Vernon Grapes earns about $22,500 annually at his electronics firm and his wife is about to give birth to their third child.

Grapes' income qualified the family for an HOC mortgage and the couple closed on a two-level town house in the Stonehedge subdivision on April 30, he said. Principal and interest are $431 a month; property taxes, insurance, fees and other costs bring the total to $517, Grapes said.

"It's just about all we can afford," Grapes said, "and maybe we don't have all the conveniences of a single-family home. But it's a nice place. Overall, I'd say we were very happy."

Area developers also like the program, despite occasional snarls in processing mortgages, they say. "It helps individuals of moderate means to buy their first house," said Stephen Eckert, vice president of Porten-Sullivan Corp., a Bethesda-based builder and developer that participated in the past three HOC mortgage programs.

Eckert said HOC mortgages have helped the firm in its goal of catering to moderate-income, first-time market in Montgomery. Two cases in point, he said, are Porten-Sullivan's Farmingdale and Waring Station developments in Germantown.

At Waring Station, a 170 town-house project, 65 units have been sold since July under mortgages HOC offered at the beginning of the year, Eckert said. At Farmingdale, 50 of 600 condominiums and town houses have been sold to first-time, HOC-backed buyers.

HOC offers 30-year, fixed-rate mortgages for homes priced less than $70,000. Waring Station homes sell for $50,000, while those at Farmingdale are $50,000 to $60,000 Eckert said.

But "there are drawbacks to the program for us," Eckert said. "It takes much longer to process loans when you're complying with the federal requirements that govern these mortgages." For instance, prospective buyers must produce copies of their tax returns for the three previous years and may have to negotiate with as many as four lenders, Eckert said. He said there is usually a minimum of 90 days to process a loan for an HOC mortgage "compared to 30, 45 or a 60-day maximum under conventional lending."

In this latest round of mortgages, HOC requires builders to pay a $1,440 "buy-down," a contribution that either lowers monthly mortgage payments or reduces closing costs, depending on the buyer's wishes. Nonetheless, "when you weigh the benefits to the buyer and to us, the program comes out ahead," Eckert said.