If you are using the C6 sleep state then it's not unusual for the core temperatures to get a little jumpy at idle. C6 lets the CPU switch to 0 volts so the difference in core temperature between that state and a normal C1 idle state can be significant. If your load temps are fine then it's nothing to worry about.

Go into the bios and turn off C3 and C6 and your idle temps won't be as cool but the temps shouldn't be as jumpy either.