NEW YORK (AP) - NBC has fallen into a deep midwinter freeze after being heartened by some long-sought success this past fall. The network has badly stumbled recently. Three of its all-time low viewership marks for a regular TV season were set in the past four weeks. The Nielsen ratings company said Tuesday the only exception came the week NBC televised the Pro Bowl. NBC had high hopes for the musical drama "Smash," but its second season premiere last week had its smallest audience ever. The medical series "Do No Harm" is gone after two weeks. The network has turned over its Friday schedule to the news division, and "Dateline NBC" was its most-watched show last week. It ranked No. 34 among all broadcast programs. CBS dominated the week in prime time.