I'm taking my i5 750 up to the area of 4 GHz, and I'm seeing a huge difference in temperature on my 3rd core. At first I was simply getting warmer than normal temps, so I reapplied my H50 using Arctic Silver 5 with the vertical line method. Now I'm getting even warmer temps, and core 3 will remain 12-13 degrees below the other cores.
Look under max load. That was my most recent Prime 95 test.
A core temp difference of 5-10 degrees is normal and nothing to be afraid of. Maybe uneven thermal paste over that specific spot between the cpu and heatsink but usually if you're not having any problems then it's fine and you don't need to worry about it.
I've learned on the Corsair forums it might also be because the heatsink is too tight. If the CPU is slightly bent, which isn't unheard of, loosening the screws might help distribute the pressure. I'll try that later on.