Alienware 17 R2 slow and laggy

Hello. For a year ago, my alienware started to feel a bit nosy, it started to become much slower and it started to lag in certan ''heavy'' games. Alienware is obviously a gaming pc, and should not get stutter lag when playing games. For over a year soon, I have my self tried to fix this problem, by doing just about anything that the internet has to offer to fix my problem. Sadly, nothing has changed anything, and its getting worse as times go. I am most surely there are something inside my pc that isnt working correctly, I am at the moment point towards my the Ram, but I cant be sure.

'' indepth information ect'' 

Alienware 17 R2 specs with,

upgraded harddrive: Seagate firecuda gaming sshd 2tb

and GPU: EVGA gtx 1060 6gb (Pluged in an Graphics amplifier)

With these specs as far as I know, my pc should handle all games like a charm, and it does, just the few minutes I am in the game. After some time, it will start lagging, stutter lag if im not wrong. The games I am spesific pointing at the moment is Battlefield 1 and GTA 5, which both is pretty heavy based games. I run them both in perfect 60 fps and above untill the lag happens, the fps drops each time the stutter happens. I uploaded a video on youtube so you can see what type of lag I am talking about ''  '' If you watch the video at around 2 mintutes, you will see this lag at its worst. Not only does the game stutter, but the audio cuts as well. 

Now my problem is kinda weird, sometimes the lag isnt that bad while other times its horrible.. Some times the auido cuts, some times not, ect... Lag like this is 100% game breaking, if im trying to do something in the game, it often wont happen becasue of the lag. Say that I want to shoot, I click the shoot key but it nothing happens, becasue of this lag, the shot didnt happen or is delayed.

Besides the gaming part, the pc is in general very slow. I have some sort of a 100% disk usage problem, which I have done some things to atleast decrease the problem. insted of constant 100% diskusage, it rather spikes to 100% from doing really nothing, but its not as bad as constant 100%. You would think  it is the harddrive then, but sadly after changing the harddrive to a better one (sshd) nothing changed, so it got to be something else.

Now I wont even list everything I have done to fix my problem, its so much I cant remember half even, and I dont know if I should put this as a question over how to fix this or a discussion over what this lag is. I am very positive that something inside my computer is damaged, but the BIOS check tells me its 100% healthy, as well with other programs going over my computer. So I have absolutely no clue on what to do anymore.

If there is anyone out there who will read this and maybe have any sort of help, that would be very much apprechiated, just anything, even the basic or the basic things to do, just tell me and I will reply if it works or not.. Thanks

also I want to appologise for my bad english, I hope you understand me anyway.. :)