Members of the House internal investigating committee took the first steps yesterday to resolve their conflicting roles as grand jury, jury and judge in evaluating the ethical conduct of their peers.

Rep. Charles E. Bennett (D-Fla.), new chairman of the House Committee on Standards of Official Conduct, set the new tone through a series of actions.

In one action, he appointed two two-member subcommittees to conduct preliminary investigations into whether Reps. Charles C. Diggs Jr. (D- Mich.) and Daniel J. Flood (D-Pa.) broke House rules by the conduct described in recent federal criminal charges against them.

Diggs was reelected last fall after being convicted of padding his federal payroll. He was sentenced to three years in prison but is appealing.

Flood's bribery and perjury case ended in a mistrial last week when the jury could not reach a verdict. Flood has since been hospitalized for exhaustion.

Bennett explained that the subcommittees, to be headed by Rep. Richardson Preyer (D-N.C.) for Flood, and Rep. Lee H. Hamilton (D.-Ind.) for Diggs, would save time and prevent leaks. Republican members were not named yesterday.

The subcommittees would then recommend any charges to the full committee, which like a grand jury, could file a formal indictment of misconduct.

In another major change that would require House approval, Bennett said he would explore the suggestion of staff director John Swanner that a "jury" to hear evidence on any charge be picked by lot from the membership of the full House. If the member charged were found guilty, the jury would recommend punishment to the final "judge," the full House.

Bennett said he was making the changes, in part, because the Justice Department had rebuffed a suggestion that it appoint "separate independent prosecutors." He asked the Justice Department about the idea because "I am uneasy about a system where judges are also prosecutors."

Philip B. Heymann, head of the department's criminal division, said that the suggestion raised serious constitutional questions about the separation of powers between the executive and legislative branches.

Bennett's suggestions were prompted by the committee's painful, time-consuming deliberations over the past two years in investigating charges of South Korean influence-buying.

Last fall, the House rejected the committee's recommendation to censure Rep. Edward R. Roybal (D-Calif.), who was charged with lying repeatedly under oath to his colleagues.

The results left several members of the committee, including Preyer and Hamilton, disillusioned and questioning the value of continuing the committee's work.

Both reportedly wanted to leave the committee this year, but a leadership aide said yesterday, "their resignations weren't accepted."

Preyer, a former federal judge, said after the meeting yesterday that he agreed with Bennett's suggested changes. The changes also have the blessing of the House leadership, according to an aide for Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

Preyer said he was concerned that last year the committee acted first as a grand jury to charge colleagues and then heard the same evidence again in reaching a verdict of guilt.

"There may be a temptation for the members as a jury to justify their own indictment," he said. "At first blush, I like Mr. Swanner's suggestion about picking the jury from the full House."