Hi,
i`ve the same "problem". i5 3570k - Asrock Extreme4 - Zalman CNPS9900 MAX - 5x 120mm Casefans.
Idle: core #3 40° and the other 25-26° (5% coreload could not be the reason of this temperature difference)
Stressed out: core #3 49° and the other 37-38° (for example Prime95, Room temperature 14°)
I`ve renewed the thermal paste three times without any differences.
I bought this System to oc it in the next few years. But i think it is an problem cause later the hotter core will
limit it extremly. Its my fourth Core 2 Duo/Quad and i`ve never had more than 5° difference.
Sorry for my bad english.
Yours sincerely