Sometime, I get really LOW frame rate.

Model

Clevo P770ZM
GTX980m
intel xeon e3 -1231v3
16gb ram
etc

ok. Where should I start this? Usually when I play pc game, I always try to get the perfect setting to hit solid 60 fps. However, I've found out that some random time, I get performance drop about 20 ~ 50%. For example,
When I run Witcher 3, I usually get solid 60 fps, but some time, I get 30 ~ 45 fps (max), and I've not changed the setting. I first thought it was gpu trotting cause by the temperature. But no. The temperature stayed at 68 ~ 74 Celsius (155 ~ 165 Fahrenheit. For you American), which means there is no gpu trotting. The odd thing is, when I unplug the power and plug back in, the frame rate goes up from 30 to Solid 60 fps.It really bothers me like hell.... Can anyone help me? Here is my firestrike score.



Low performance


Regular performance

 

I forgot to mention that the cpu runs cool as well.