It's a classic heat error of some sort - otherwise it wouldn't recur when you stress the system. Which would make it hardware, but you've replaced pretty much everything that could be broken. Perhaps you're maxing out your power draw when everything starts running hot & fast. Have you tried a bigger power supply, ie 1000W instead of 800W?

I found this very interesting to read, since ive not often heard of illustrators agents while book authors agents seem to be fairly common. Does it happen often for an illustrator to work mostly through agents? Is it the same kind of thing?

There is a book called "nudge" which you should read. It contains quite a bit about how placement of things influence choice, and how you can architect people's situations to get them to make better choices.