Check temps.

I can't believe I forgot to do that!
http://i.imgur.com/TwjW409.png (if you can't read it they all say 95/96oc)
Ok there's something wrong.
As mentioned I cleaned out my pc recently, clearing the dust from the fan and all components and making sure it was all plugged in.

Yes, the throttling is evident in the task manager screenshot.
Have you reseated the cooler or accidentally loosened one or more screws/plastic clips?

How has your computer not exploded yet? I'm pretty sure intel cpus generally auto shutoff somewhere in the 90s Celsius

It should be put perfectly back in place after I cleaned it out, it should be plugged in as well.

Is it really that bad?

Ok double checking the CPU cooler fan, which I forgot to mention is the stock one that came with the i5 2500k.
Will be back asap.

You know that you can't just reseat a CPU cooler without replacing the thermal paste (most of the time, there are exceptions)?

No, CPUs don't explode, only bad PSUs do that. Iirc Sandy Bridge throttling starts at 93° and ends at 98° (Tj max) where the throttling becomes ridiculous, past that -> shutdown. No explosions

Most CPUs turn down their frequencies at temperatures above 70C to prevent damage. As setsul said, you should check your cooler. I had a similar situation with my old PC, and all it needed was a new application of thermal paste to fix things.

I don't know a single CPU that throttles at 70°C.

Well I think this is an exception.
I shut down my pc and detached the CPU cooler to double check it, it is perfectly clean since I cleaned it yesterday.
I placed it back in and did some small tests to see if it actually spins, it does.
Plugged everything else back in, put the cover back on and rebooted.
An BIOS message came up regarding changes made to CPU / Memory? I went through it and I didn't see anything dumb so I discarded the changed and continued the boot.
Looking at Core Temp now the temperature is now at a solid 30-36c for each core. No idea how it's suddenly fixed itself.
http://i.imgur.com/8rUeYTI.png
I'll play a game of Tf2 and see if anything weird happens. I'll let you know asap
Thanks for the help anyway Setsul! :)

One of the clips probably didn't slip in all the way the first time so the cooler didn't get proper contact, but enough for cooling idle/throttled. I'm speaking from experience.

Can I hop in with a cheeky hijack? My PC is bluescreening loads recently and always in cs:go, used the big windows debugger and the bluescreens points to 'GenuineIntel' which I'm pretty sure comes down to fault in CPU. I've had this PC for a few years now, it's an i7 4770k at 4.5 (been stable for years, never had an issue till a few weeks ago)

Most of the things I looked at told me it was either temperature based or faulty cpu, but it's idling around 35 and looks to be around 55 when playing. Says max was 70. As far as I'm aware this shouldn't be shutting down my PC at that temp.

Any advice? Cheers lads.

It's been stable for 2 years or less. How stable? What tests did you run? Run the same tests with the same settings for the same duration again and see if it's still stable or if the BSODs are very frequent go back to stock and see if it stops bluescreening.
Voltages? Cooler?

Also did you know starting new thread is very easy and free? Use the green button "Start Thread" on the top right.

In all seriousness though, don't do this. This is not a general help thread. It's a different problem so it doesn't belong here. If I had mod rights I'd be cutting out all these posts and moving them to new threads with a big red mod edit disclaimer in the first post everytime this happens.

I'm reviving this thread again because 9 months later I've got basically the same problem.

I was away for 3 nights on a short holiday, my pc has been off since Monday and I just arrived today.

I just turned my computer on again and then it seemed to have the same issues as the 9 months ago, being the slow processing speed but not high CPU usage. I presumed it was the fan but right now it appears to be working.

I have absolutely no idea why my CPU core temps are so high now. Is it time for a new sink?

EDIT: Ignore, just had it extremely loose for some reason.

Try remounting your backplate for the cooler? Never seen a cooler just randomly come loose, even the older shoddy plastic tab coolers.

I'm actually taking the opportunity to actually upgrade my stock cooler which came with my i5 2500k, I'm getting the Cooler Master Hyper 212 Evo.

I have that cooler and so far I haven't had any issues with it, just make sure you clean off and re-apply the thermal compound because if there isnt a good enough contact with the paste and the heat spreader then that can also cause some issues, the EVO 212 also comes with a small tube of thermal compound if you dont have any.

I have mounted this cooler multiple times to both intel and amd pcs.
It does a good job when you actually get it set up, but getting it set up is a massive pain in the dick.

Just as a heads up; make sure you get an extra pair of hands to help you when you mount it if possible.

#16
MY CRUSADE AGAINST THE 212 EVO BEGINS ANEW

http://skinflint.co.uk/raijintek-ereboss-black-0r100011-a1163291.html
Literally the same price as the 212 Evo
http://skinflint.co.uk/cooler-master-hyper-212-evo-rr-212e-16pk-r1-a684731.html
but better
https://www.techpowerup.com/reviews/Raijintek/Ereboss/

And that's just the 2nd cooler on the long list of coolers that perform better than the 212 Evo and are the same price or cheaper except when it's on sale for 20-25$ in the US.

EDIT: Another one
http://skinflint.co.uk/be-quiet-pure-rock-bk009-a1184606.html?hloc=uk
https://www.techpowerup.com/reviews/beQuiet/Pure_Rock/

I could go on.
Or name a budget and I'll name a cooler.