Question Pc laggy while playing games

Sep 3, 2019
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I dont know if i am in right section but nevermind.
I bought brand new pc at local shop.They installed me windows 10.
When i came home i started up my pc
When im moving my mouse in the sircle its lagging sometimes.
Im playing gta v for like 30 minutes in 120 fps and suddently its droping on 45 and it wont go up,just laggy.
Even if i leave the game its still laggy until i restart pc
Ryzen 5 2600x,1660,16gb ram 2400mhz,a230,600w,1tb,240gb ssd