THE DOOMSDAY CONSPIRACY, by Sidney Sheldon (William Morrow: $22; 400 pp.). A manly yet sensitive hero, governmental cover-ups, wanton violence, UFOs and a tacked-on save-the-planet message--what else could you need? Realism? Pshaw. Even by Sidney Sheldon standards this is weak, with more than the usual number of holes and silliness plus a concluding twist that is easily guessed by the fifth chapter. Navy Intelligence Comdr. Robert Bellamy's top-secret mission is to locate all the witnesses to a mysterious incident without access to names or any details. Busy mooning over memories of his perfect marriage with his ex-wife, which seemed to consist solely of great sex, Bellamy barely reacts to the phenomenon he uncovers. The epilogue boasts of extensive research into UFOs; too bad little of that effort went into creating a plausible, solid plot.