It could be - cpu cooler dirty, cpu cooler not good enough, cpu cooler needs re-seating with new thermal paste, cpu is overclocked with Vcore too high, mobo voltage not controlling cpu voltage, power supply voltages wrong, cpu fan rpm too low.
Post your full and detailed build - every component make and model plus OS - per forum guideline.
Download and run HWInfo - Sensors then post the results - usually 3 screenshots in TinyPic or similar.