WHEN ASKED LAST WEEK whether he believed Dutch journalist Willem Oltmans' testimony to the House Assassinations Committee, Rep. Richardson Preyer (D-N.C.) replied, "Ask me that question again in a few months." It was a good response. The committee cannot keep others from popping up with stories of conspiracies against President Kennedy or Martin Luther King Jr., as Mr. Oltmans did with his vague accounts of talks with George de Mohrenschildt, a former acquaintance of Lee Harvey Oswald who apparently committed suicide last week. But the House panel does not have to react instantly to such developments, and indeed cannot do so without sacrificing all hopes for a systematic, responsible and trustworthy inquiry.

The next few months should show whether the reconstituted committee has really settled down after the series of misadventures that made its first months so embarrassing to the House. It now has a new chairman, Rep. Louis Stokes (D-Ohio); an opportunity to engage a new chief counsel; a prudent set of rules and investigative procedures; a more sensible $2.7-million budget, and an authorization from the House to carry on for the rest of the term of this Congress. With its lease on life much more secure, its members should be better able to resist the temptation to court headlines and toss out tantalizing scraps of "evidence" to justify the inquiry.

The panel has been criticized for presenting little so far except a mishmash of old gossip and tired tales that have already been discredited. There is a lot of such stuff floating around. But there are also persistent questions about each case that trouble many people who are not wedded to any particular explanation or enamored of conspiracy theories in general. These questions - such as how James Earl Ray's travels were financed, and why the FBI was so intent on proving that Lee Harvey Oswald acted alone - should be the focus of the investigations now.

Perhaps the trails have grown too cold; many people who might have been helpful have died; memories have become undependable; vital evidence may have been destroyed, or may be hidden beyond reach. In view of all this, the committee should not inflate public expectations by holding back its findings for some grandiose final report.Instead, as partial conclusions emerge from the investigation, they should be set forth in periodic public hearings or sharply focused interim reports. That way, the public can gradually get some answers about both the cases and the committee itself, and realistic judgments can be made about the wisdom of pressing on. After all, the final answer may turn out to be that there can be no final answers. The committee ought to reconcile itself, right now, to the possibility that this could be its maximum contribution to public understanding of these tragedies.