Unusual CPU usage and performance

Hi there.

I've got a i7 4770k OC'd to 4.20Ghz

A couple of months ago my cpu fans (Corsair H100i cooler) started constantly ramping up to 60 - 100% when I was playing Fallout 4. I just assumed Fallout was just really badly optimised.

Turns out this trend continued with other games, but more worryingly, other simple tasks like browsing the web - or downloading a game on steam.

Here's an image of Task Manager showing 100% usage, but some RainMeter desktop widgets showing no more that 20% on each core. So that's my first query. Why is that so?





The fans do clearly need to spin up as my CPU is regularly maxing out at just about 100 degrees! What can I do to try an diagnose the problem. Up until Fallout 4, My PC ran like a dream. Silent and relatively cool, now it's a nightmare if I have to do anything even relatively taxing. It still runs fine, but makes an awful racket, and pushes a lot of air around the place.

I've tried replacing the thermal paste on the CPU twice (Arctic Mx-4) - both not better than before.

I don't know what's gone wrong...