PSU: make, model, wattage, age, condition?
Use Task Manager and Resource Monitor one at a time to observe system performance.
First while just "idling", then by doing non-gaming activities, and finally while gaming. Three tests.
Just drag the window to one side or the other and position to watch.
See what happens or changes as you go through each of the three tests.
And remember that the purpose is to find the problem and not necessarily win the game you are playing. Try to discover what happens in the games to cause the fps drops.
You may need to "lose" to do so.