Scan with malwarebytes free version. Malwarebytes scans for different things than Norton. It might not find anything (but it almost always does when first used), but it is a very handy program to have.
https://www.malwarebytes.org/mwb-download/ Read carefully as you install. I don't think they will try to load extra stuff (not like them to load extra stuff) but be alert anytime you install anything. Their software is trustworthy. Just download and then execute. It will download their database and do a scan. It doesn't leave anything running in the background when it finishes. It only runs when you ask it to.
Ignore anything regarding upgrading to their pro version, for now. You don't need it
If it finds anything, you can choose to let it take care of it, or just ignore it for now and report back here.
Nothing you report makes me think malware, but
Malwarebytes is just good, uncontroversial, computer maintenance. BTW, you can get malware just sitting on a site with ad servers. You don't have to click on anything. I had a computer rooted by a hacked ad server, through a popular news site, while no one was at the computer.
again, regarding the symptoms:
1. Can you just open a bunch of files and get it to occur reliably?
2. Need to know CPU and Memory usage when the problem occurs
3. Turn off Norton when the problem occurs. Better yet, if you can make the problem occur, turn off Norton and then try to make it happen.
Regarding swapping:
Your system has a certain amount of physical memory, and a disk file call the paging file or virtual memory (some call it a swap file) where it can store data that isn't being currently used by the running programs. When you run out of memory it will swap some older program data to the disk to make more room.
You should check how much swap memory you have.
Control Panel -> System -> Advanced System Settings -> Advanced Tab
under performance, press Settings -> then Advanced
Under virtual memory it will have a number. Mine is 6.135GB (or 6135MB)
You can click on Change and set it to let the system decide if it isn't already set that way. Or increase it, or whatever. Don't decrease it.