RIVERSIDE (CNS) - The average price of a gallon of self-serve regular gasoline in Riverside County dropped today for the 32nd time in 33 days, falling four-tenths of a cent to $5.401, its lowest amount since March 13. The average price has dropped 66.4 cents over the past 33 days, including 1 cent Sunday, according to figures from the AAA and Oil Price Information Service. It dropped 18 consecutive days, was unchanged June 8 and resumed decreasi…