Social Security's antiquated computer system lives on the edge of a data-processing disaster. "Our problems are very severe and they directly affect our ability to provide services to the 36 million beneficiaries," Social Security Commissioner John A. Svahn said in an interview last week. "We make thousands of mistakes every month, and in many instances we're quite concerned as to whether or not we can actually issue those checks."

Among the many problems listed by the commissioner:

* Accidents. "Not a month goes by that maybe we print the zip code where the check amount is supposed to be," he said. Social Security has recently been testing the computer program that will pay beneficiaries their 7.4 percent benefit increase in July. Some of the test data got mixed up with the live runs for the May checks. As a result, Social Security put out some cost-of-living adjustments two months early this year.

* Out-of-date checks. If you retired on Social Security but continue to work at a part-time job, your current Social Security check may be too small. The money you earn every year, when added to your account, should raise your basic benefit by a small amount. But Social Security's cumbersome computers have been so loaded down with other work that no benefits have been recomputed since 1977, except for the cost-of-living increase. This year, Svahn is updating checks through 1979; you'll receive any money you're owed for those years.

* Tardy enforcement. If you're 65 to 72, and have been earning more than $6,000, your Social Security check may be too big. Your basic benefit is supposed to be reduced by $1 for every $2 earned over that amount. But Social Security has not been running any enforcement checks to see who is over the limit. (For people under 65, the earnings limit is $4,400.)

Enforcement runs are now being made for 1978 and 1979. Beneficiaries who received higher payments than warranted will have that money withheld from future Social Security checks.

Out-of-date earnings records. The W-2 forms that you attach to your income tax returns are sent along to the Social Security administration, which posts those earnings to your Social Security account. But for hundreds of thousands of accounts, Social Security is up to four years behind.

Most earnings information eventually reaches the proper account. But occasionally W-2s are lost or contain errors which prevent them from being posted to the correct accounts. If such a thing happened to you, it would have the effect of reducing your future Social Security payments.

Check up on your records by asking Social Security for a free copy of your current earnings statement (request forms are at your local Social Security office). If your records are wrong, the local office can help you put them right.

* Routine changes. "People will go into a Social Security office and fill out a change-of-address form," Svahn said. "It may or may not get processed in a timely manner. The check may or may not be sent to the old address, or the new address, or either address--even two checks to both addresses."

Social Security also finds it hard to respond when a child grows up and goes off the rolls, or when a beneficiary dies. "In some instances of death, families had tried to stop the checks and failed," Svahn said, "so they were just holding onto the checks." When people keep quiet about the death, and spend the checks that continue to come, Social Security prosecutes for fraud.

Social Security got into this mess by failing to spend the money needed to keep its computer system up to date and its personnel well trained. The agency has had a rapid succession of commissioners, none of whom stayed in office long enough to do very much about its problems.

John Svahn has proposed a five-year program for modernizing Social Security's data processing system, which he says will cost about $500 million. He thinks the administration will give him the money for the job--but won't say how long he intends to stay at Social Security to see his proposals through.

Next: Checking up on your earnings