You need to observe Task Manager or Resource Manager (just one or the other - not both at the same time) while first not gaming and then while gaming.
It does take time to learn to navigate the tools and get a sense of the information being provided.
Observe system performance in three ways: 1) immediately after boot while doing nothing. Then 2) drag the window to one side or the other and observe while doing light work or browsing. Lastly, 3) while gaming.
Look for some bottleneck.
Try to find something that you can do to "control" the fps drop. Or discover something your computer starts doing and the fps drops. Maybe a backup or AV scan.....
Work to understand what is running on your system; perhaps launched during...