IT'S THE RATINGS, STUPID: White House officials are intent on unveiling President Clinton's much-awaited health care reform proposals with a major speech to a joint session of Congress in mid-September. But their plans to deliver the speech on Sept. 22 have run into a serious roadblock: sitcoms. . . . Sitcoms, cop shows, video magazines--it's the new fall lineup from ABC, CBS and NBC, and the three commercial networks are not at all happy about having the debut interrupted by a news event, according to a senior Administration official. . . . Traditionally, the networks air major presidential speeches, but no law compels them to do so. Concerned that the big event might be preempted in prime time, White House officials, led by presidential counselor David Gergen, are busily negotiating with network executives. Stay tuned.