There are tools to stress the hardware, but I'm not sure of a way to stress a system the way you are thinking, except pen testing.
Are you following best practices and not allowing Admin access (I would probably create Guest accounts, which are even more limited)?
Are these computers intended for web-browsing, or just general use?
Have you looked into EMET?
8.1 has many usability improvements over 8. I would theorize many of your issues would be solved by updates. Frankly, not applying critical updates at minimum is negligent. There are tools to manage updates, but you are essentially saying you won't give your kid medicine, because he is sick every time he needs medicine. Correlation is not causation. Follow best practices first and then troubleshoot.
One other thing which piqued my interest is what are you using for Anti-virus? Are you aware Windows 8 has AV built-in?