Trying to conserve cash, AOL Time Warner and the Walt Disney Corporation have revived long-running talks about combining their CNN and ABC News operations into a joint venture in an effort to cut costs, people involved in the talks said yesterday. Both companies are more motivated to reach an agreement than ever because they are suffering through a prolonged downturn in advertising sales while under pressure from investors and credit-rating agencies to pay down their debts. The discussions also take place at a time when the top executives of both AOL Time Warner and Disney are under particular pressure from shareholders and directors to improve their results. The talks reached a new level of seriousness about three weeks ago, only to founder over questions about the size of each side's stake in the possible joint venture and the degree of each side's editorial control over its news. The revival of the talks was reported yesterday in The Los Angeles Times.