Three years isn't old, certainly not for a normal domestic installation. I've never experienced problems due to lack of cleaning. I do clean regularly, but only to avoid overheating and consequent component damage. I've opened much older cases that had never been cleaned and were black and furry around fans, with heavily-clogged heatsinks, but the computers were still performing OK. Not convinced that's the problem, but do it anyway.
Do you have any monitoring programs running while playing, say for testing FPS, temperature, CPU usage and such? Don't.
Before playing a game, run Autoruns (autoruns.exe, download from Microsoft's site) and stop as many things from starting up as you are certain won't cause problems - msconfig.exe isn't as comprehensive. Then do a reboot, stop your antivirus, and then start a game.
I agree with James Denison that with 16GB RAM you should not be experiencing much, if any, disc access. So running tests on each stick is good advice. It will also be worthwhile running Chkdsk and surface tests on your disks, and do Defragment the drives. As for the amount of space remaining, you may need more. So uninstall and delete what programs and files you don't need. There are lots of discussions concerning free space requirement. Here's just one -
Also, go to ControlPanel\System\Performance\Settings\Advanced\Virtual memory\Change and make sure both disks have Paging set on them, and maybe try increasing their sizes. Then try disabling Paging altogether as it forces Windows to only use RAM, and you certainly have enough RAM to at least experiment. Discussion here -
When you do solve the problem, do return and explain what did it.