Well I would say if the Cpu sensor on the motherboard is at 80c +
God knows what the actual sensor of the cpu die package is reporting.
As it is bound to be higher than what the external sensor is reporting if part of the motherboard.
It`s no wonder it will be throttling the cpu it`s self is likely to be in the 90c + range if the case.
A badly fitted cpu cooler in most cases.
And if you are using the Intel stock cooler that comes with the Intel cpu.
And it uses the push pin type attachment of the cpu cooler when fitted to the motherboard by pushing down on them.
It normally means they are not fully pushed down and through the holes to securely lock and latch to the underside of the motherboard through the four holes the push into...