massive lag spikes - help

ive recently build a pc its pretty decent and it runs every thing mint, but recently after a couple of minutes of playing beam ng it start getting lag spikes every 30 seconds or so but if i change application then go back to the game it will be gone for a couple of minutes , any one know how to fix
no other games do this

Motherboard :H170a gaming pro
Processor Intel(R) Core(TM) i5-6600K CPU @ 3.50GHz
Video Card NVIDIA GeForce GTX 960
Memory 16 GB ram
Operating System Microsoft Windows 10