I honestly believe that it's your card. Sure, that PSU isn't going to win any awards but the fact that it's 80+Bronze-Certified means that it has to be at least decent and 600W is more than enough for your system specs. I agree with DSzymborski about overclocking a GPU that old. It's a crapshoot and the odds aren't in your favour.
I have a philosophy when it comes to buying PC parts and that is to never buy a part that you'll want to overclock immediately. If you can't get the performance you want or need from a part at stock, you shouldn't be buying it in the first place, you should be getting something better.
Overclocking is best when used to extend the life of a product so that you have time to get its replacement.