"Sleepless in Seattle" is trying to loosen the firm grip of "The Firm" on the rental chart's No. 1 position, but no luck so far. At just about any other time, "Sleepless," starring Tom Hanks, would own the top spot. But there are just too many Tom Cruise and/or John Grisham fans in the rental market. "Dave," the Kevin Kline comedy, also has No. 1 potential that may never be realized because of the competition. It climbed two more notches to No. 3 but will have trouble moving any higher. The sun appears to have set on "Rising Sun's" chances at No. 1. It peaked at No. 2 and could never sneak past the December chart leader "Cliffhanger." Two newcomers--"Heart and Souls" (No. 14) and "Menace II Society" (No. 18)--should do strong rental business.