System sometimes slows down. RAM or CPU or GPU issue?

[keiraija]

My system will sometimes slow down in performance (in both graphic and CPU performance).
I test it using 3Dmark11, sometimes it will be very slow in physic test (2-3 fps when slow down, 20fps when its normal, the loadtime is also slower)
Huge drop in graphic test
I use a memtest to test the speed, and it takes around 60ns to response when normal, but when it slows down, it will take 150ns to response
The problem will most likely disappear when I restart (sometimes have to turn off) and then turn it on

My guess here is that my timings is low (or voltage? it's rated at 1.65V at 2400, but I run at 1600, so I adjusted it to 1.5V) and it makes the RAM unstable which leads to the whole system sometimes slow down? (bus 1600 8-8-8-24 1.5V)
Or could it be the graphic card or the CPU which causes such problem?

EDIT: oh, and do note that when the system slows down, my CPU will only load at around 50%, even during a CPU stress test using IntelBurnTest

My rig:
Intel Core i5 4460
G.skill F3-2400C11D-8GXM
Gigabyte B85M-D3H
Gigabyte GTX 970 G1 Gaming
Cooler Master V650W

[Alidanexei]

Perhaps you have malware or adware on your system. Other than that, I'm not really sure what could be slowing you down.

[Astralify]

Check your temps. The CPU could be overheating and throttling. Maybe the cooler is not seated properly. When did it started occurring? Could also be malware.

[keiraija]

Check your temps. The CPU could be overheating and throttling. Maybe the cooler is not seated properly. When did it started occurring? Could also be malware.

 

All temps checked (IntelBurnTest, Prime95 for CPU peak at 73c / Furmark,3Dmark for GPU peak at 74c) - pretty good even when benchmarking in its "not slow down" situation

Virus/Malware checked using Avast/Windows Defender - nothing found

RAM checked (8-8-8-24) using memtest - passed

 

It's hard to track down what causes this because it doesn't always happen (but very regularly)

[AliveGhost13]

Try just putting the RAM on the XMP profile, so it should run without problems as that's what it's designed for..

Try with one RAM stick, then if that doesn't help the other stick, also try in different slots to what you are using now