I'm leaning toward a bad temperature sensor on that core, or the program you are using is reading the data incorrectly. On my quad core even if I am only loading one core hard the other 3 are usually within a couple degrees of it.
Try using another program to monitor the temps or look in the bios(if this is happening while the cpu is idle) and see if the same thing happens. If so, you may also want to try reseating your heat sink.