I import a quick time video file on AE. It does not play back like the orginal file. I have a person talking in front a green screen. Every few seconds the person fades in and out, he's there one second, disappeared the next, again and again as if flashing. Why is this happening? How do I fix this? It does not play like that on the original, just when I import it into AE
Try updating or rolling back your graphics driver.
Or using AE CC 2014, where such odd behavior occcurs far less frequently. It also wouldn't hurt to do a purge of all memory & image caches.