"NEW YORK -- Under attack by its own listed companies and powerful floor traders, the New York Stock Exchange is considering reinstituting a ""collar"" on program trading that it abandoned last year, according to people familiar with the Big Board.</br></br>The exchange also may step up its disclosure of firms engaged in program trading, these people said.</br></br>Big Board officials wouldn't comment publicly. But in an interview in which he called the stock market's volatility a ""national problem,"" Big Board Chairman John J. Phelan Jr. said, ""We are going to try to do some things in the short intermediate term"" to help the situation.</br></br>Mr. Phelan has been viewed by many exchange members as being indifferent to stock-price swings caused by program trades. He said he is ""very surprised"" by the furor over program trading and the exchange's role in it that has raged in recent days.</br></br>Mr. Phelan said that the Big Board has been trying to deal quietly with the issue, but that banning computer-assisted trading strategies entirely, as some investors want, would be like ""taking everybody out of an automobile and making them ride a horse."""