... including the PSU manufacturer, model, wattage, and how old it is, RAM model and speed etc.
I highly doubt you will need 16 GB RAM paired with your current specs - the fact that this is a recent thing and not something happening all along proves this. Same goes with CPU upgrade. If it was a CPU too slow, then why would it have only started 7 months ago?
Having FPS drops that last a specific amount of time suggests it is a background process takijg a chunk out of your system's resources. So, you are going to want to do a few things to try to stop this.
Firstly, download
https://www.malwarebytes.com/ and do as full system scan, as well as leave all realtime scanning on during the 14 day free trial. The reason why is suggest Malwarebytes is because it finds lots of nasty stuff that other antiviruses don't even consider to be malware, or skip over. Delete or quarantine everthing Malwarebytes finds.
In addition this, if you use optimisers, driver updaters, speed boosters etc, then uninstall them if Malwarebytes doesn't remove them itself. If you were using them, then, uninstall all drivers ('uninstall a program' from Control Panel), and get the latest drivers straight from the official websites. Plus, you should make sure they didn't damage any Windows files. Open the start menu, and search for CMD. Right click command prompt and choose run as administrator, then allow it. Type "sfc /scannow" (without the "). This will take a long time. Allow it to finish.
Next, right click the bar thing at the bottom of Windows, and choose Task Manager. Look at the startup tab, and disable anything non-vital. Do not disable Nvidia, Steam, Sound drivers, etc. Only things that you dont need. If in doubt, just ask whether you should disable it.