I've had an i7-3930K for roughly two years. 2-3 months ago, the performance began to severely degrade, with no change in hardware or software.
I noticed that the Core temperatures are idling around 75C. However, the motherboard is reporting the CPU temperature at 45-50C.
While doing anything stressful, such as gaming, the Core temperatures increase to over 90C and the frequency is throttled to 1200 Mhz. At the same time, the motherboard's CPU sensor reads 45C, which is even lower than it was while idle. I suspect less heat is being generated because of the throttling.
If I run Intel's Processor Diagnostic tool, the motherboard's CPU temperature goes up to 80C, and the Core temperatures remain in the 90s. This leads me to believe that the motherboard's sensors are working correctly.
I've also re-seated my heat sink, confirmed it's functioning, and I can feel it transferring heat.
I believe the Core sensors are reporting incorrect temperatures and causing throttling. Is there any way for me to diagnose this, or a solution if this is a known problem?
For all measurements, I've confirmed that SpeedFan, ThrottleStop, hwmonitor, and ASUS's tool are all showing roughly the same numbers.
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.