Sudden drop in fps

So I sometimes play World Of Tanks on low settings with a stable 60-80 fps and suddenly it's gone down to 15-25 fps. This has happened before and it somehow went back up to 60-80 but I don't know what I did. Please help as I can barely play any of my games anymore!

My Dxdiag is attached
 

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Games that you play with 'problematic game play'?

use hwinfo or speedfan or cpuid hardware monitor and post temps, voltages (12v+, 5v+, 3.3v+ ratings).

list specifications of your computer (processor, motherboard, memory, video card, storage).
 
You know what happened?

It fixed itself!


(The reason I couldn't reply straight away was because I was on holiday for a week and I'm going on holiday yet again for another 2 weeks so I probably won't be able to reply, sorry)
 
It's come back again so yeah...
It does effect other games so that's true.
Um I'm not a computer expert so if you could tell me how to find out all that stuff above then that would be great, I tried getting a new PC but my parents said £1500 was too much, xD
 
Quick update, I just noticed that my own charger couldn't actually charge my computer. All it would say was 'plugged in' not 'charging'. So I had to use my dad's charger and his was able to. Could this be a big issue?
 
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