Complaints over alleged widespread overcharges on adjustable-rate mortgages intensified last week as attorneys filed two new class-action suits against federally chartered lenders.

The suits, part of what lawyers and lending-industry experts said will be a nationwide campaign of litigation this fall and into 1991, seek redress for all adjustable-rate mortgage borrowers at two Indiana-based savings institutions.

Other class actions in New York and as many as a dozen other states, according to lawyers, are expected to be filed in the coming weeks. Last week's complaints, similar to a class action suits filed Aug. 10, allege patterns of extensive overcharges at the periodic adjustment intervals of borrowers' loans.

Officials at Permanent Federal Savings and Shelby Federal Savings were not available for comment on the complaints.

About 12 million American homeowners hold adjustable-rate loans. Mortgage experts who have studied lenders' portfolios say anywhere from 25 percent to 50 percent of them carry incorrect computations. Total overcharges outstanding on the 12 million adjustable-rate loans, they say, could exceed $8 billion.

John M. Geddes, a computer consultant who studied 7,000 adjustable-rate mortgages for the Federal Savings and Loan Insurance Corp., reported that the monthly payments demanded by lenders were wrong in half of them.

The Federal Deposit Insurance Corp. and the Resolution Trust Corp. both are studying adjustable-rate loans under their regulatory purview to determine how serious the overcharge problem is. Lending-industry officials dispute Geddes's figures and insist that the issue is being blown out of proportion.

M. Scott Barrett, a Bloomington, Ind., attorney who has been co-counsel on the first three class actions filed in his state, argues that banks and S&Ls have "plenty to gain" by facing up to the enormity of the problem and rectifying it.

"Half of the mistakes we've seen {by examining potential plaintiffs' loan files} result in undercharges to the homeowner," Barrett said last week. "They {the banks and S&Ls} are losing money on their ARM {adjustable-rate mortgage} mistakes and they have no legal way to collect it."

Under the federal truth-in-lending act, consumers may recover money from erroneous overcharges by lenders. But federal rules prohibit lenders from doing the same when their computation mistakes result in undercharges to mortgage customers.

Barrett, who's been investigating the adjustable-rate issue for clients for more than a year, listed the types of "commonplace" errors he has found by examining loan files at lending institutions in New York, California, Illinois, Oklahoma and Indiana, among others.

Topping the list: Improper use of loan indexes, including timing errors, substitutions of one index for another and improper selection of the index date for computing the new adjustment.

One client took out a one-year ARM tied to Treasury notes, with the first annual adjustment scheduled for Aug. 1, 1986. Yet in 1986, 1987 and 1988 the bank changed the rate effective July 1, not Aug. 1 as the loan contract specified.

"To add insult to injury," said Barrett, in 1988, the bank unilaterally changed the index from one-year Treasuries to the 11th District Cost-of-Funds (a measure based on the cost of funds at S&Ls on the West Coast). In 1989 and 1990, the bank changed the rate effective on Sept. 1, not on Aug. 1."

"The borrowers' only comment," he added, "was that 'we assumed the bank was doing it correctly. We trusted the bank.' "

Seemingly innocuous index switches like these can skew rates in the lender's favor, and cost borrowers large sums -- sometimes $1,000 or more -- in overcharges over the course of a year, Barrett said.

The federal truth-in-lending law allows aggrieved consumers like Barrett's clients to band together as a class to sue for repayment of their overcharges, plus attorneys' fees and costs. The statute set a ceiling of $500,000 or 1 percent of the lender's net worth as the maximum award per class action.

Among the tip-offs to possible overcharges on your own adjustable-rate mortgage:

Any transfers of "servicing" or ownership of your loan from one lender to another during the past several years. Many computer botch-ups apparently occur while moving your data from the existing system to a new one.

The index used on the loan is not a "garden" variety, regularly published in the financial press. An example provided by Barrett: One client's one-year ARM was indexed to the "2 1/2-year average Treasury yield curve." Good luck finding that one in any financial publication, he said.

Your adjustable loan is more than five years old. Analysts say the early generations of ARMs have the highest probabilities of payment computation errors.

Your lender can't adequately explain the latest adjustment to you. When the loan-servicing staffers themselves haven't the foggiest idea as to how your new monthly payment was arrived at, you know you're in shoal water. If there's even the slightest hint of overcharge, it may be worth your while to seek legal advice.