Vice President Al Gore skewered the competition for “Late Show With David Letterman” on Wednesday, while Fox Broadcasting Co.’s “The Chevy Chase Show” took a 28% fall in its second outing.That combined with the primetime premieres of “Beverly Hills, 90210″ and “Melrose Place” to create a good news, so-so news night for Fox, as “90210″ zipped off with strong numbers and “Melrose” scored its best rating since July 15, 1992, when it aired its second episode opposite three-network coverage of the Democratic National Convention. CBS’ “Late Show” had its best results this week with Gore’s appearance, pulling a 6.6 rating, 21 share in Nielsen nationally compared to a 4.1/10 for “Chase” (which starts a half-hour earlier, at 11 p.m.) and 3.0/9 for “The Tonight Show With Jay Leno.” In their common half-hour, from 11:30 p.m. to midnight, Letterman doubled up on the competition, posting a 7.3/20 to a 3.6/10 each for the rival talkshows. The Fox entry took a 22% dive in its second half-hour, only slightly higher than the anticipated fall-off Fox has sold to advertisers — a 5 rating from 11- 11:30 and 4 rating during its last 30 minutes. Still, Fox execs pointed out that the show performed well in key demographics , including a 3.2 rating among men 18-49 compared to a 1.4 for “Tonight” and Letterman’s 5.1. As for primetime, “90210″ zipped off with a 12.4/21 to win its hour in both households and key demos, with a 26 share among adults 18-49. The show was up sharply in rating over last season’s premiere. “Melrose” (10.4/17) fell 16% off its lead-in and garnered a 20 share among the key adult demo. ABC previewed its new Wednesday lead-off show, “Thea,” and generated some hefty sampling, posting a 13.6/22 despite fumbling 19% of its “Home Improvement” lead-in. It’s noteworthy that the Castle Rock comedy jumped 3 share nationally above its metered-market tally, a rare occurrence for ABC. NBC’s “Now With Tom Brokaw & Katie Couric” (11.3/18) held its own in the 9 o’clock hour but yielded the younger audience, relying on adults 50 or older.