S&Ls brace for new round of challenges. US savings-and-loans and British building societies have similar histories: financing homes. Today, both have similar problems, but different answers.

The other night, Jonathan E. Gray was watching a movie on late-night TV. It reminded Mr. Gray, an analyst at the brokerage Sanford C. Bernstein & Co., of the industry he follows. ``The history and future of the savings-and-loan industry reads like a Marx brothers script,'' he says. ``There's inadequate regulation, inadequate accounting; there are even crooks.''

Of late, it has been the crooks that have been getting most of the attention. Fraud has contributed to half of the S&L failures since 1984, according to government regulators. Last week the government closed down one of the most notorious thrifts, Vernon Federal Savings and Loan in Texas. Vernon's management - which had bought a fleet of planes, a corporate yacht, and a beach house for the majority stockholder to live in - has been accused of making risky loans and using fraudulent practices to fuel the company's 1,000 percent growth over five years. The government is injecting $1.3 billion so the thrift can reopen under new management.

But the fraud problem, while still worrisome, is waning, says an aide on the House Banking Subcommittee. ``The civil litigation, the stockholder suits, the criminal charges'' against the management of questionably run thrifts ``have been thrown up as a warning for other thrift managers,'' he says. ``People consult the statutes these days.''

Now banking experts say that the country is heading into the next phase of problems in the thrift business. Those problems have been pushed out of the spotlight by the stock market collapse and moves to revamp the banking industry, says William Haraf, a financial economist at the American Enterprise Institute. But, he says, they are every bit as important.

``It's politically easier to focus on re-structuring issues rather than the hard questions about where we're going to get the money'' to bail out troubled thrifts, he says. ``We need to put that on the front burner right away.''

About 10 percent of the country's thrifts are insolvent; 20 percent are on the Federal Savings and Loan Insurance Corporation's (FSLIC) problem list.

And bigger problems loom, says Gray at Sanford C. Bernstein. If interest rates rise - as they did before the Federal Reserve pumped in money to buoy the stock market last month - then many thrifts will be in trouble, he says.

For one thing, he says, many borrowers have taken out adjustable-rate mortgages. Those people could see their monthly payments rise by 30 to 40 percent over three years, Gray says.

In addition, only a third of the interest rates on loans (income for a thrift) are adjustable every year, whereas 70 percent of deposit interest rates (a cost for a thrift) are adjusted every year. So when interest rates rise, thrifts' costs jump faster than their income.

During the last round of interest rate hikes (1978-82), the number of thrifts fell more than 15 percent: Some failed, some merged with other thrifts. Gray expects about the same percentage to fail or merge over the next five years.

To manage the problem, this fall Congress gave FSLIC the green light to raise $10.8 billion for the fund.

That's not nearly enough, banking experts say. It will cost an estimated $40 billion to close the thrifts now known to be in trouble. Vernon's $1.3 billion bailout alone will take more than 10 percent of that $10.8 billion.

Government regulators have tried other solutions, but with limited success. For example, the Federal Home Loan Bank Board's ``management consignment'' program would let it seize control of a troubled thrift and hand it over to a management team borrowed from a well-managed thrift. But 2 years and 60 thrifts later, the management teams are still trying to rescue their charges.

Meanwhile, other solutions are being tested. For example, a bank may buy a troubled thrift if no other thrift makes an offer.

The Federal Reserve Board, however, may have thrown cold water on this solution. In September, it proposed letting banks buy not just troubled thrifts, but healthy ones as well.

The United States League of Savings Institutions, the industry association, is trying to stop the plan, fearing the ``bankification'' of its industry. The league says banks would buy up healthy thrifts and then - when a moratorium is lifted next August - switch the thrifts from FSLIC to the Federal Deposit Insurance Corporation, where the premiums are lower because the FDIC is stronger. That would leave only the most troubled thrifts in FSLIC, the league says.

M. Danny Wall, the new head of the Federal Home Loan Bank Board, which regulates thrifts, sugests letting a bank buy a healthy thrift if it also buys a troubled one.

Mr. Wall also supports the idea of ``megathrifts,'' in which perhaps five or six thrifts would combine under one management. That would give weaker institutions access to more capital and - more important - sophisticated leadership at the top, something widely thought to be scarce in the industry.