WASHINGTON (AP) _ The Internal Revenue Service pledged Friday to try to improve its new W-4 tax-withholding form, acknowledging the document is so confusing it could cause many Americans to doubt that the tax system has been overhauled.

''We are moving heaven and earth to address the problems'' with the W-4, IRS Commissioner Lawrence B. Gibbs told congressional panels. ''I hope in the next several weeks we will be able to come out with something,'' especially for lower- and middle-income taxpayers. March 1 is a reasonable target date, he said.

However, Gibbs would not promise flatly to revise the form - or more specifically, the instructions and worksheet designed to make it easier for taxpayers to claim the correct number of withholding allowances. The simpler the W-4 becomes, he said, the greater the chance a taxpayer will have too much or too little withheld from paychecks.

The commissioner did promise to search for a way to avoid imposing penalties on taxpayers who make an honest effort to have the right amount of tax withheld but fall short because of problems with the W-4.

In the meantime, Gibbs recommended:

-Continue filing the new form. Otherwise, the taxpayer runs the risk of being overwithheld (giving the government free use of the money) or underwithheld (perhaps having to pay a penalty a year from now).

-If the federal tax withheld from a worker's paycheck this year is significantly less than was taken from the last check received last year, the employer should be asked to take out at least enough to make up the difference.

Gibbs testified for 90 minutes before the Senate Finance subcommittee that oversees the IRS and then moved to the other side of the Capitol for a session with a House Ways and Means subcommittee. Lawmakers let him know how upset they are about the new W-4.

''This really does a job on the American taxpayer,'' Sen. John Heinz, R- Pa., complained.

''A veritable hailstorm of protest'' has resulted, said Rep. J.J. Pickle, D-Texas, who heads the House subcommittee.

''The W-4 form ... has started a prairie fire across America that is leading to a taxpayer's revolt,'' said Sen. David Pryor, D-Ark., chairman of the Senate panel. He reminded Gibbs of the biblical story of Zaccheus, the despised tax collector who later repented and became a disciple of Jesus.

''Mr. Commissioner, today we want you to repent ... We want to offer you the opportunity for salvation,'' Pryor said.

While not repenting, Gibbs did acknowledge that the confusion about the W-4 works against the goal of having taxpayers adjust their withholding to reflect major changes brought about by the 1986 tax-overhaul law.

''If this is the result, then our efforts toward accuracy (in having the right amount withheld) are less productive, and in the larger picture we risk having the public's perception of tax reform negatively affected,'' he said.

As part of the big tax overhaul last year, Congress directed the IRS to produce a W-4 form that would more nearly accurately permit taxpayers to balance their withholding with the tax liability. In striving for that accuracy, Gibbs conceded the instructions and worksheet have become more complex.

The form itself, the one-third of a page that the worker gives to the employer, is basically the same as it has been for 20 years. The almost two pages of instructions and worksheet have become almost four.

Accountants, tax lawyers and many ordinary taxpayers have complained to Congress about the complexity. Treasury Secretary James A. Baker III, whose department has authority over the IRS, also said the form is too confusing and ordered the agency to simplify it.

The IRS is testing one proposed new form but already is concerned about its accuracy, Gibb said.

''My guess is that when we come out with a new form ... we are going to find it will be a form that, in order not to have unacceptable levels of underwithholding or overwithholding ... will not be usable by all taxpayers,'' he added.

That means that while the great majority of taxpayers may get a simpler worksheet and instructions, the affluent and those with complex tax situations likely will have to live with the present form.

Gibbs said he agrees with the intent of legislation that, for 1987, would roll back a portion of the new law and require taxpayers to pay in advance, through withholding or quarterly payments, at least 80 percent of expected taxes. The new law raised that level to 90 percent.