fps decrease in half by restartring FSX not Windows

i have noticed a decrease in half of fps running FSX in Windows Vista after closing fsx and then restarting it later on without rebooting the computeris this a normal phenomenon or am i doing something wrongdo other people experience the same drop in frame rates after restarting fsxby the way if i restart windows vista by rebooting the computer my frame rates return to normal in 20-30 fpsi am running clean fsx with sp1 without any scenery add ons uder Dell XPS700 with E6600 Core 2 Duo, 4Gb Ram and 512gb 7900GTX video cardany suggestions or help or thoughts would be appreciated Bo