CPU flickering between 30 & 100% utilization

Hi, my PC has a i7 2600 quad core cpu, since I purchased it in April its been perfectly fine.

In the last 2 days my Media Encoder renders have been really really slow, allowing the problem to run a 4 minute render took 6 hours

Before the problem surfaced I would use the "details" option in task manger to set the after effects background process priority to real time, same for ME

I now have to run it on normal because my CPU has decided to die on me somehow and slows my pc down by cycling between low usage and 100% !!!

Other pc specs are GTX v and 16gb RAM