I'm not a tech support guy, but it sounds like a (memory-related) buffer overflow of sorts. Your memory, RAM, becomes full, and it starts using your hard drive as a place to store temporary files needed in order to run your computer. This usually affects the whole system drastically, depending on your setup, and can choke the performance down, as you've described.

Assuming this would be the case, do you remember updating any motherboard- and/or memory management-related drivers (like RAID, SSD, graphical) recently? Another possibility is that some program running in the backround is hogging up resources all of a sudden, like antivirus-software that starts running a full-scan, let's say, on a hourly basis (surely it's highly unlikely, but i've met a few cases were it was 'accidentally' set to do that, for reasons unknown)? Some recently installed programs may have an inconvenient auto-updater firing up, but... looking at your description it'd had to be one huge m-f'er to choke it down that badly.

Maybe more important questionwould be, do you hear any more extra noise inside your computer when this occurs? Does the fans pick up more speed, or does the hard drive start making a rapid lowish, ticking/crackling noise?

Considering how ruthless this game can be to components, i really suggest you pop open the side of your PC, and check that you don't have any dust clogging your cooling systems. Some setups (usually depending more on the driver- and user-set options) may lower automatically your PC's performance, in order to run cooler. Would kinda make sense if your comp was full of dust by now, and cannot run as cool anymore, as it has ran before.