Try reseating the chip. It could just have a bad connection leading to a miscommunication between the motherboard sensor and the on die sensors. Also check what voltage the chip is getting and compare it to the processors data sheet. If its high, lower it back some. What cooler are you using?
You also stated that performance degraded over time. Run prime95 with small fft for 5 hours to see if the chip is stable. If the system bluescreens or the chip shuts down to prevent damage, you may have a problem with the cpu.
Keep me posted.