Internal Revenue Service computer and operational problems are more widespread than previously thought, and more than 150,000 taxpayers may have received erroneous dunning notices from the troubled Philadelphia Service Center alone, according to two new government reports.

Some area taxpayers who filed early in the tax season are experiencing unusually long delays in getting their refunds and, in a twist, taxpayers who waited longer to file are likely to get their refunds first.

The delays were caused by the failure of the $103 million computer system to record many returns the first time they were put through the system.

Delays for those refunds are running 12 to 16 weeks at the Philadelphia center, where returns from the District and Maryland are sent, a spokesman said.

That means a taxpayer who filed Feb. 1 might not get a refund until about May 20. The IRS will not begin paying interest on late refunds until June 1, however.

The number of long-delayed returns is not known.

The "normal" lag for returns this year is 10 to 12 weeks for Philadelphia and slightly less for most other centers. About 530,000 returns reached the Philadelphia center before Feb. 1, but spokesman Daniel Seklecki said there is no way to count how many had to be processed a second time.

The earlier returns came "when we were having our greatest amount of problems" with the computer, Seklecki said.

"That's why a lot of people who filed early have not received their refunds and those who filed later have."

The reports, by the General Accounting Office and the IRS' internal audit division, focused on problems nationwide and especially at Philadelphia. It is running behind most centers and appears to have had more trouble than some others in adapting to the new computer.

The Philadelphia center frequently has dunned taxpayers for money they do not owe and has generated reports of lost or shredded returns, although IRS officials deny allegations that thousands of returns have been destroyed deliberately.

The internal report said some taxpayers still have not been officially notified of the error last fall that failed to record withholding-tax payments for 26,756 corporations in the mid-Atlantic region. Thousands of delinquent notices were sent and levies placed on five bank accounts as a result of that error.

This was thought to be the only failure of its kind.

But the GAO report disclosed that there have been other such errors at the Philadelphia center, affecting as many as 150,000 additional taxpayers who may have received erroneous notices, levies and liens.

At the end of March, the Philadelphia center had a backlog of more than 100,000 pieces of unanswered correspondence from taxpayers.

The Philadelphia backlog was the greatest in the nation, with 89 percent of letters more than 45 days old.

The only one of the 10 centers with a larger correspondence inventory was the Memphis center, where tax returns from Virginia are filed.

It had 114,000 piece of correspondence on the shelf, according to the GAO. Memphis generally has been one of the faster centers in processing returns and refunds.

Each of the Philadelphia foul-ups has caused other difficulties. The withholding-tax failure, for example, generated 10,000 letters of protest, which worsened the center's backlog.

And thousands of calls from angry taxpayers have inundated the center, overloading the Philadelphia telephone network.

IRS spokesman Scott Waffle said the service could not comment on the GAO report because officials did not have a copy. An IRS staff report accompanying the internal audit study agreed with many of its recommendations for preventing future errors such as the withholding-tax snafu.

The two reports were made available to the The Post by the oversight subcommittee of the House Ways and Means Committee, which is to hold hearings today on the adequacy of the IRS budget request for fiscal 1986. Congressional critics, such as subcommittee chairman J.J. (Jake) Pickle (D-Tex.), say they are worried that proposed budget cuts will increase communications and collections problems even after the new computer is running better.

For fiscal 1986, the administration is asking for 86,489 staff positions, fewer than the service had in fiscal 1980 and almost 9,000 positions fewer than the IRS sought before its budget request was cut during the administration's budget-drafting process, according to congressional sources.

Congressional pressure on the IRS also is intensifying from other quarters.

Rep. Doug Barnard Jr. (D-Ga.) has accused the IRS of "grossly inadequate planning" for installation of its new computer, including underestimating the time it would take to train employes to use the system and choosing a computer language inappropriate to the type of computer.

Individual horror stories about the IRS also continue. Shirley Rowland of Alexandria, for instance, was expecting a refund of $348 this year.

Instead, she got a bill from the IRS for $15,508.06. She said she has sent copies of her original return to the Memphis center but has had no reply and no confirmation her certified-mail package was received.

Also, the Baltimore Evening Sun reported that Carnell and Norma Jean Williams of Lansdowne were told by the IRS that they owe $2,106,394.63 in back taxes, interest and penalties. Norma Jean Williams was unable to get through to the IRS Baltimore office to explain the mistake because the phone was always busy.

Among the GAO's findings on the Philadelphia center:

* Thirteen magnetic tapes with records of 116,000 1984 quarterly tax payments were processed three months late, and the IRS district offices were not notified by the center that taxpayers might be calling about erroneous notices.

Another computer tape was temporarily lost. It* contained records of 30,000 withholding tax payments from the second quarter of 1984, and delinquent notices went out to all 30,000 taxpayers before the mistake was found.

* The failure of the center's taxpayer-service operation to respond to taxpayer correspondence may be due to a procedural change in which generalists handle all kinds of cases. In the past, IRS employes in the correspondence branch were divided into specialities and answered only mail involving their expertise.

* The computer network that provides access to taxpayer accounts was working only 26 percent of its scheduled hours in 1984 and 18.6 percent of its scheduled hours during March.

*The computer is not "turned off" when a taxpayer says he has been incorrectly assessed. As a result, taxpayers can continue to get computer-generated delinquency notices while they are explaining their situation to a human being.