Pc lags and stutters after long uptime

[Avi Assayag]

So i have a decent pc (amd fx 8350, 16g of ram, gtx980ti, Asus sabertooth MB). But lately 1/2 year or so my pc starts lagging horribly after being up for 2 hours or more. I mean unmanegably slow;
The mouse randomly gets stuck (stutter but not freezing), videos are running intermittently slow (after about a minute it stutters and lags and then goes back to normal just to start the cycle again in a minute). Even in games I have massive dps drops every minute from 70ish to 0 for a second and then it comes back to normal. When it happens only hard reset can make it go away.
I've tried running scan for viruses (Kaspersky & malwarebytes) - all clean and tidy.
Complete disassemble of pc for cleaning and maintenance including applying new thermal paste on CPU. Swaped a different gpu (amd 7970) and installed drivers with DDU, both on amd and on the nvidia card. Still the same. And then,  I did a complete format reinstall of OS after I swapped dual channel ram sticks in the MB trey. Nothing helped. I have a Samsung was with no paging file (I had one but I still had the problem for months). Ram test was fine.

I'm loosing my ABSOLUTE shit here guys :-(

Any suggestions as to why it happens and what I can do about it?

 

[RARRAF]

PSU ??

 

[Avi Assayag]

evga 1300watt 80+ gold standard

 

[Rocky Bennett]

I run Windows 10 and leave my system up for weeks at a time, so 2 hours is just too short of time to cause any issues.

 

[Avi Assayag]

what can i do to explore the issue?

 

[RARRAF]

try some benchmarks see what is slow https://www.aida64.com/downloads

 

[Avi Assayag]

thanks for the advice. I've run memtest for the night with all programs closed and in 16gb capacity. in the morning I saw no errors report in the app. meaning that maybe it is not RAM related, tough it sure as hell feels like it does :-(

I'm out of my depths here really