heatsink fan not working but started spinning after 2 hours of gaming/usage

[coolkmmr]

So I was playing then I realised that I was lagging. So I checked my CPU temps and saw it was 100 C. I checked everything and when I removed the side panel, I saw that my heatsink fan was not spinning. So I decided to put external cooling by using an electric fan. After for 2-3 hours of gaming, It was getting kinda cold so I removed the electric fan and to my suprise the heatsink fan was working and my cpu temps were back to normal. Can someone explain how did this happen and I need a perma fix. Thanks

 

[jojesa]

How old is your system?
Which cooler do you have?
Could you post your system components.

 

[coolkmmr]

How old is your system?
Which cooler do you have?
Could you post your system components.

System is almost 5 years old
If you count intel stock cooler lol
CPU : Intel I7 4700k @ 3.50 GHZ
MOBO: ASUS H81M-C
RAM: 8 GB
GPU: GTX 1050 TI GALAX

 

[jojesa]

Your cooler fan is about to completely fail. You might need to replace that Intel cooler.
There are several units like these you could use.
-Cooler Master Hyper RR-T4-18PK-R1
-Cooler Master Hyper 212 Evo.
-DEEPCOOL GAMMAXX 400
-be quiet! BK009 Pure Rock