Game Not Responding

First off, when I download the game, it only gives me a 400 mb download instead of a 900 mb, and i think this may be the problem of the following: I have an AMD 7870 Radeon Graphics card, windows 7, 8 gigs of ram, etc and nothing is faulty. Although when I run the game, it keeps saying (not responding) then 5 minutes later, it responds. then I go to click something, and it does it again. I have tried to run other games to see if the problem still persists, but it didn’t. I have verified game cache, reinstalled twice, and nothing seems to be working. Any help out there?