Q: Apps (including Finder) frequently stop responding

I'm on a Mid-2010 iMac with 8GB RAM and the latest version of El Capitan.

In the last couple of days, my iMac has begun to frequently stop responding. The problem doesn't seem to be app-specific; it even happens when I'm running nothing but Finder. I'll be in the middle of a task, and suddenly everything stops. I get the spinning beach ball, and often Activity Monitor will display the current app as "not responding." After a period ranging from a few seconds to a minute, my iMac snaps out of it. Then, a minute or two later, the same thing happens again.

What have I changed recently, you ask? Mid-June I installed new printer software, and upgraded to Office 2016; however, both worked fine for a couple of weeks before this new problem surfaced. App Store doesn't show any significant recent updates, and nothing at all in the last five days.

Activity Monitor doesn't offer any clear answers—there's no memory pressure, and the CPU idle ranges from 70-98%.

My startup drive's SMART status is fine, and Disk Utility's First Aid spots zero issues.

I've also gone through the predictable easy fixes—restart, reset SMC and NVRAM.

What else should I be looking at, or trying? Is it worth reinstalling my OS? Or might this be a (gulp) hardware issue?