Associate
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1) Keep the "Task Manager" open whilst you are using your PC and see if any processes are hogging your CPU the next time its jerky.
2) Run the Windows Experience Index (I know its not a great benchmark, so no flaming pls). This should give you a score between 5 and 8 for the main components in your PC - if you get any 1s or 2s then it could help to narrow down the source of the problem.
No unusually high processes in the task manager, and my scores are 7.5, 7.6, 6.7, 6.7, 5.9. :/
Those WEI scores are good scores, even the 5.9 which is the maximum for a mechanical drive.
Wrt task manager I meant keep it open all the time, with the "Always on Top" option ticked. The moment you experience any jerking around, look and see if anything is using your CPU and/or Memory under the Processes tab and also see if there are any spikes on the Performance tab.
Good luck