Disgruntled investors taking complaints against stockbrokers to arbitration have been winning their cases with increasing frequency since this mode of dispute settlement became common.

But an analysis of arbitration awards indicates the trend in favor of customer claims may be at a turning point."There's some indication that the customer `win rate,' which has been rising gradually over the past few years, is starting to moderate," said Richard P. Ryder, editor and publisher of Securities Arbitration Commentator.

The Maplewood, N.J., newsletter monitors the arbitration field and periodically reviews awards to spot trends. The latest analysis covers awards rendered by arbitrators in the securities industry's self-regulatory organizations from May 1989 through June 1992. Arbitration awards started becoming publically available about four years ago.

Two causes may account for the slowdown in the rate at which customers prevailed in arbitration proceedings.

Ryder said broker-dealers probably have tried harder to settle cases before they go to an arbitration panel. Totally unworthy claims and ones involving unquestionable wrongdoing increasingly are being resolved without arbitrators getting involved.

"Cases are grabbed early and settled. The really bad claims rarely make it to hearings," he said.

That leaves the arbitrators to decide cases in which customers might have a tough time proving their arguments, Ryder said.

A second cause - considered less plausible by Ryder - is that more frivolous claims are being filed. "I tend to think it's the first (cause) and I hope it's not the second," Ryder said.

Yet the sheer volume of filings and the hyperbole used by some attorneys acting on behalf of claimants suggests that cases lacking merit are finding their way into this forum intended to give investors a say.

This past week on Wall Street, investors turned their attention to corporate financial reports, several of which had a big impact on stocks of individual companies.

Motorola, for instance, enticed buyers with rosy fourth-quarter figures that surpassed estimates of analysts who closely monitor the chip maker. The stock jumped 1 to 99 on Friday.

Stock trading continued brisk and volume on the New York Stock Exchange was again above 300 million shares Friday - as it was for eight out of the past nine sessions.

The Dow Jones industrial average on Friday more than made up for its 23-point loss over the prior three sessions and reached a new closing high in the process. The Dow surged 24.77 to 3,867.20 Friday, surpassing the previous high close of 3,865.51 set only last Monday. The index wound up the week with a 46.43 point gain.

Other market measures reached new highs.

The New York Stock Exchange composite index rose 1.17 to 262.90, just above Monday's record close of 262.76. It added 2.56 in the week.

The Nasdaq composite jumped 4.50 above Thursday's record close to another new high, 792.31, and ended 9.37 higher than a week earlier.

The Wilshire Associates Equity Index, the market value of NYSE, American and OTC issues, ended the week at $4.728 trillion, up $51.214 billion from the week before. A year ago, the index stood at $4.314 trillion.

The American Stock Exchange market value index edged 0.82 higher to 480.88 and ended the week 1.39 ahead.