I'm running an i5-3570k at stock speeds with a Cooler Master Hyper 212 Evo and Arctic Cooling MX-4 thermal paste. I've noticed that there is a 10-11 C temp difference between cores 1 and 3, both while idling and under full load:
Idle (minimum temps):
Full load (maximum temps - prime95)
I've heard it's common for individual cores to have different temps, but is mine normal? Even with the difference, should I be fine for gaming since 67 C is the hottest I've gotten with prime95?
UPDATE: Zero issues after two months. I've put it through heavy gaming sessions (including recent games like Far Cry 3) and hours of video editing without any problems. It's been completely stable.
Funny because I am experiencing exactly the same thing! ~10C difference between cores 1 and 3. Core 1 always seems to be relatively the hottest core. I was starting to worry a little but I guess as long as all temps are within a good range, I won't focus on the core temperatures so much.
By the way, I am using the Hyper TX3 with the MX-4. Running at 4.2Ghz (-0.065 offset) I am getting more or less the same temps you are. Never over 68C after an hour of P95).