System slows down if I leave it on overnight

That sounds like a memory leak, it's caused whenever a programmer calls a pointer to a new object in a loop and forgets to delete it after each iteration. So in other words it's not a hardware problem it's a problem with your software. Start task manager in the morning and check your processes to see which program is eating up your memory then uninstall it or remember to terminate that program when you shutdown.

Make sure to update your anti-virus and do a full scan. Also use malwarebytes free edition. Also, depending on your system it may be better to turn off the system rather than put it in hybernate or sleep.

I'll test these out and get back to you