GTA V, PC Issue?

[JBG123]

When I first played GTA V I was able to run settings on a mix of high and very high decently, this lasted for two days. After two days I was getting a bit of lag here and there so I lowered my settings to only high, this lasted for one day. After one day I was getting all sorts of stuttering and lag issues I lowered my setting to normal. If I stand still in-game I get 70-80FPS but If I try walk it drops anywhere between 40-60 and if I try to drive it goes mental about 30FPS is the average. I'd say my PC was pretty decent does anyone know why this is happening?

My Specs are:
Gigabyte 990FXA UD3
Kingston HyperX FURY 8GB (2x4GB sticks)
AMD FX 8370 Black Edition
MSI R9 390 8G
WD Blue 1TB Desktop Hard Disk Drive
SAMSUNG SSD 850 EVO 120GB

NOTES:
I have GTA V on the SSD.
My Drivers are up to date.
I'm using a 1440p monitor.

I forgot to mention I have upto 5.8GB's of memory free when playing GTA V is that normal?

 

[P4-630]

For some reason I need to alt+tab first in GTA V to get smooth gameplay.

 

[JBG123]

For some reason I need to alt+tab first in GTA V to get smooth gameplay.

That used to fix things when I first got it but now it doesn't do anything, also If I alt tab out sometimes it will make my PC lag.

 

[P4-630]

What are your CPU and GPU temps during playing?

The GPU and/or CPU could be throttling.

 

[JBG123]

What are your CPU and GPU temps during playing?

The GPU and/or CPU could be throttling.

GPU ~ 70°C
CPU - 30-35°C

 

[P4-630]

Did you solve the problem?

 

[JBG123]

Did you solve the problem?

I didn't solve it but I found out what it is.

Those last to values used to be 4GB and 2GB so GTA V wasn't getting enough ram.
After hours of trying all the methods suggested by windows and forums, I found out it was a hardware issue. I am getting new specs so I haven't bothered to see if it was the Mobo, Ram or CPU's fault.