Help Please!!! Oblivion 4 playing choppy all the sudden where do i start?

I have been playing Oblivion 4 for 3 months no problem but all the sudden as i move through the scene things get poppy. Not slow framerates just no longer the fluid motion it once had. I have turned down the settings and also as low as 640x480 and still the same "poppiness, jumpiness and my character moves through the scenes. Everything in distance scenes which once appeared gradually and fluidly now just pops into view. Where do i start? Is this natural? Do i need to reinstall the game-want to know if so ill reorder it ive since lost the other disk...help!!