"One core" is the highlight here. Unfortunately you probably have the problem I also experience. With Ivy Bridge, Intel moved to a cheaper TIM under the IHS to save money. Some of the chips, like mine and probably yours, have a bad TIM application which makes the temps uneven and is probably the reason one of your cores is getting so hot. You have a few options:
1. Get a new chip and pray it doesn't have the same problem.
2. Under volt and down clock the chip, also find a program that manages single thread performance (so that it spreads load out across several cores) and can improve temps up to 5c or so.
3. De-lid the CPU, get CoolLabratory Liquid Pro and apply to die and IHS, then re-lid, reinstall CPU cooler. This method is the most effective but also takes technical know-how and delicate care. There are guides out there how to do this and it has results up to a 20c improvement.
Or maybe you didn't use enough Arctic 5 on your primary installation. A rice sized grain is not enough for a water cooling block. Use the line method and make a thin line that covers 60% of the height of the CPU.