"Platoon" fought off potent competitors for the second straight week to cling to the No. 1 position on the Billboard magazine rental chart. "RoboCop," which appears to have No. 1 potential, hasn't been higher than No. 3. "Beverly Hills Cop II"--out next week--also looks like a candidate to take over the top spot. "Spaceballs" is up to No. 8 in its second week on the chart and could make the Top Five. Other debuts: "Revenge of the Nerds II" (No. 14) and "Nadine" (No. 16). "Lady and the Tramp" still heads the sales chart, which measures sales to retailers and distributors.