Yeah, I'm not anxious to go to El Cap, given all the bad reports, so I'm holding off for now. I have not thought about re-installing 10.9.5 - originally my iMac was on 10.6 (Snow Leopard) which I loved, and hung on to for a long time... until I finally went from 10.6 directly to 10.9. I tend to stick to an OS for a long time if I find it works for me - I did the same thing with my first Mac - I waited until Tiger was out before I got onboard, and I loved Tiger. I then waited until Snow Leopard and stuck to that until like 10.9.3 or so (don't remember the exact 10.9.x). I saw heard a lot of bad stuff about Leopard, so I skipped it, same for Lion, also skipped it. Didn't like what I saw of Yosemite, was hoping El Cap would be better... but then thought I'd skip El Cap too - until my recent Copy & Paste troubles.
There is one other possibility - my HDD might be failing, because sometimes there are other odd behaviors, like freezes or really a long time to launch an app, or the app randomly won't launch at all. Stuff like that. My HDD is a Seagate 1TB - and I've always had problems with my HDD on this iMac - I had the HDD replaced 3 times under warranty (and AppleCare) - all Seagate drives. Maybe bad luck Seagate struck again, who knows. Personally I don't like Seagate drives, because I've always had bad luck with them even as external drives, had at least 4 external HDD fail all Seagates.
If it is the HDD that's bad luck as I'm out of warranty completely - I had 4 years altogether... year 1 basic warranty year 2 and 3 Apple Care, and year 4 was from American Express (because I bought the iMac on my AmEx card). But now I'm completely out of coverage, and so would have to pay for a HDD replacement myself - I was actually thinking that if the HDD fails, I might tackle the swap for a new one myself. Really, I was hoping this iMac would last me until such a time as Apple came out with an iMac that's tempting - and I think it is close, what with a high resolution monitor on the 27"... just hoping for a bit more, and if all goes well, maybe I can buy a new one come 2016 or 2017. I want another 1-2 years out of this one (which is a late 2009 model I actually bought February of 2010).