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 startup or launched by you plus what resources are being used by each application, process, or service.
As for the PSU - if it is indeed only 1.5 years old it may be fine. However (depending on the make, model, and wattage) the PSU may not be up to the power demands of the games you are playing. And a low end PSU is more likely to start failing early in its' designed life span.
Another thought: check update history - maybe while you were taking a break some update was installed or applied.