X51, 2 years old, severely lagging randomly

I have bought my X51 almost 2 years ago and it was working fine until recently it has been lagging severely. Most times it is not straight away. It will take about an hour before it starts lagging and when I say lagging, I mean really bad lag. As in I cannot play games or watch videos because the stutter. I have run all the diagnostics I possibly could and did not find a problem.  I have tried removing my drivers and re-installing them all but to no avail. I have also updated to the latest bios and the problem still occurs. The only problem with live support is the problem has a delay of about an hour before it occurs so I am really stuck on what to do. Looking forward to any reply. There is a DXDiag attached with all my specifications.

An example of my problem =
I will play for an hour on 1000 fps easily and then it will suddenly drop to about 30 and stutter and the audio will stutter because of the lag and it will become unusable Until I restart it. Which then fixes it for another hour.