I can't say about the pc, but I have it on the PS3 and I don't have the problems you are talking about. I just downloaded episode 4 last night and haven't got to try it yet, but it has been fine so far.
I played through half of episode 4 last night. I still had no visual problems. If this is the only game giving you problems, you may want to uninstall it and redownload the game.