A stockholder's suit questioning the sale of Flow General Inc. stock by corporate insiders has compounded the problems besetting the McLean-based research firm and manufacturer of biomedical equipment.

The suit, filed in chancery court in Wilmington, Del., by a stockholder named Milton Small, raises questions about the disposition of stock by Flow General President Joseph E. Hall when he was preparing to warn stock analysts of probable depressed earnings.

Hall confirmed through a spokesman yesterday that he sold 8,002 shares of Flow stock for about $135,000 less than a month before gloomy earnings projections were made at a meeting with analysts on May 4, sending the price of the stock tumbling.

Hall sold his shares during April at about $17 a share. The stock closed yesterday on the New York Stock Exchange at 7 7/8, down 5/8 from Friday's close and far below the 45 3/8 mark the stock reached last year when the company received a National Cancer Institute contract to develop the drug interferon.

Hall declined to comment yesterday on the suit or on his sale of the stock. Douglas H. Poretz, Flow's director of corporate communications, said a Wall Street Journal story reporting the stock sale was accurate, and referred questioners to the comments Hall made in a long interview he gave the Journal last week.

In that interview, Hall said he sold the stock to raise cash to pay taxes, without regard to the company's performance. He said he still holds about 126,000 shares and was "absolutely flabbergasted" by the continuing decline in their value.

He acknowledged, however, that he had begun to receive reports as early as March that Flow General, which had been predicting good results for the first quarter of this year, was actually experiencing a "tough quarter" in which depressed earnings were likely.

Federal regulations restrict corporate officials from benefitting from information that is not available to the investing public. Hall told the Wall Street Journal that securities dealers and investors have raised questions about his stock sale, but he said, "If I could turn the clock back to April 1, I'd do what I did."

On May 4, after Hall had sold the stock at $17 or more a share, he stunned securities analysts at a meeting in Baltimore by saying that earnings reports for the quarter ended March 31 would show a flagging performance, which they, in fact, did. Earnings for that quarter were $389,000 (5 cents a share), down 80 percent from a year earlier.

Wall Street analysts expressed their disenchantment publicly, and they apparently still are cautious about a stock that was once a very hot ticket. Peter Stearns, who follows Flow General for Wheat, First Securities of Richmond, said yesterday that, "If you focus on the company and can't find anything more wrong than what has already happened, the stock will be worth a buy," but not at a price higher than 6 1/2.

Stearns' comment about "what has already happened" referred to a series of legal and financial problems that have swirled around Flow General in the past year.

In May the Securities and Exchange Commission charged the company with raising money from investors without revealing that it planned to use some of it to acquire an Italian firm. Flow settled the case, without admitting wrongdoing, by consenting to a court order requiring it to abide by securities laws.

The same month, a Flow subsidiary, General Research Corp., pleaded guilty to federal conspiracy and corruption charges relating to a contract to develop computer software for Army personnel records. Charges against five company officers were dropped, but they resigned. The Army has since notifed General Research that it may be barred from bidding on future contracts.

In June, another stockholder, William Weinberger of New York, sued Flow in federal court, alleging that a 1980 stock offering was made at "contrived, manipulated and inflated prices." That offering raised $27 million, some of which could be jeopardized if Weinberger and others who bought the shares win their case.