GTAIV Mazda 6 MPS ENB Series patch 6

Hello this is my Mazda 6 MPS mnodel for GTA IV with ENB series SORA only for patch 6 PLs help, i had min FPS 11 max 30 avg 20 when i had all very high. When i have all to high or medium my FPS is min. 13 max 35 avg 24 . Why i had little FPS ? thanks for help Intle Core i5-750 Quad-Core GIGABYTE P55-UD4 KINGSTON 4GB Turbo DDR3 1800MHz Zdroj Gigabyte ODIN Pro 550W, GE-M550A-D1 ATI Radeon HD 5870 1TB WD Caviar Black 3.5“, SATA 300, 7200 rpm, 3Gb/s, 32MB, Black edition case : XG Viper 3