Adjustable rate mortgages, which have gotten a lot of attention for the headaches they cause borrowers, are turning out to cause headaches for lenders as well.

Tracking the indexes, changing rates and payments, notifying borrowers and, of course, collecting monthly payments -- all the functions that are know in the industry as servicing -- are a lot more complicated and expensive for adjustable mortgages than for fixed-rate loans.

Most lenders so far haven't been able to figure out what these extra costs add up to, but it seems certain that they ultimately will find their ways into the rates borrowers are charged for ARMs.

"We don't have a good handle on that yet," said Richard E. Knapp, chairman of Landmark Savings in Pittsburgh, "but I'm sure they're considerably more expensive to service. They've got to be."

Many lenders already are adding an eighth of a percentage point to their ARMs to cover the additional costs, but most candidly admit they have no idea whether that is enough.

As a general rule, lenders in the past added three-eighths of a percentage point to the loan rate to cover servicing. With fixed-rate loans, the lender's cost is usually less than that, so he can make a profit. Mortgage bankers, who sell their loans to investors to get new money for more loans, get much of their income from servicing.

Indeed, there is enough profit to make servicing contracts valuable commodities that can be bought and sold in much the same way loans themselves are. Several billion dollars' worth of servicing contracts have changed hands this year alone.

But adjustable mortgages plainly involve a lot more work for the servicer.

Where once a mortgage banker or savings and loan had only to recalculate the escrow account once a year and send out a new packet of coupons, a company servicing adjustable loans has to keep track of every index that is used in any of its loans. It has to keep track of when the loans come up for adjustment, make sure that the borrower is notified in advance -- making sure that computers don't forget weekends and holidays -- and then, of course, it has to make the actual adjustment.

Computers are essential.

"If it wasn't for IBM and Lotus 1-2-3, I don't know what we'd do," said Jerry A. Bills, president of Fidelity Federal, a small savings and loan in Galesburg, Ill.

But even computers are not enough.

A roundtable discussion of ARMS servicing at the Mortgage Bankers Association of America's annual convention last month in San Francisco drew a standing-room-only crowd. Participants cited everything from postage costs to the fact that "you've got to hire smarter people" to keep track of everything.

One speaker noted that his firm is owned by a big bank-holding company, and its marketing department is constantly thinking of new wrinkles for loans that are not always compatible with its computer programs.

"They'll market anything the lawyers will approve," he said. One type of loan involved negative amortization -- where unpaid interest is added to the loan balance -- with no interest on the negative amortization amount. "They got 300 of those out before we caught 'em," he said, and now, even though his company has "the most sophisticated computers around, we have to service every one of those little devils by hand."

And even when the loans are standardized, the adjustments may not be.

Suppose a lender has sold 1,000 loans to an investor, and interest rates climb, allowing a big upward adjustment, Thomas R. Harter, head of the Mortgage Bankers Association's economics department, said recently. The investor may not want to take the full adjustment on several hundred of the loans because he would risk throwing those borrowers into default.

Then the servicer is faced with having to do all of these separately, and his computer may not help. Transactions "are only program- mable if they are routine," Harter said, and partial adjustments are not routine.

Harter warned the mortgage bankers at their convention that their race to cut costs through computerization may make them less able to adapt to a changing market. Large investments in computers may make them reluctant to introduce loans that don't fit their programs.

At the same time, pressure to cut costs on variable- and fixed-rate loans is intense.

"We used to send a monthly bill and receipt plus a franked envelope," Bills said. "Those days are gone. Still, until the end of this year you get a monthly bill and receipt, and envelope and no franking. Starting in January, you get 12 coupons and 12 stickers you can put on your own envelope."

Many lenders view personnel costs as a major potential problem with ARMs. One at the mortgage bankers' roundtable remarked that while 15 percent of his firm's portfolio is adjustables, they generate 40 percent of the phone calls to his customer service department.

"Used to be, the guy went away, and that was his payment and you forgot about him," said Herschel Rosenthal, president of Flagler Federal Savings in Miami. "Now, he's going to be back in there. You're changing his payment, both up and down. Number two, most people don't understand it. So you know you're going to be talking with him. He's going to come in."

This and the other factors add costs, "and in the end it's going to get built in" to the price of the loans, he said.