In ''The Rat Pack Captured,'' a recently rediscovered tape of a live filmed television performance from June 1965, Frank Sinatra, Dean Martin and Sammy Davis Jr. are frozen in time or as they might have put it, swingin' through time. Entertaining and revealingly dated, the program captures the Rat Pack members almost minutes before they became caricatures of themselves. Three months later, Dean Martin began his long-running television variety show. In a few years, Mr. Sinatra made ''My Way'' his boastful theme and begin to build his legend. In 1965 you can see the mannerisms and routines that would become exaggerated signatures. Those styles have been clouded by years of parodies; here is a chance to see a straightforward original. Called ''The Frank Sinatra Spectacular,'' the show was staged in St. Louis to benefit a halfway house for ex-convicts and was broadcast on closed-circuit television around the country.