The FPS drop might be because your computer gets overheated? I mean, if I start bot today, going to do 24/7 for a few days, I'll check my computers current temperature, it's usually 30-40. When I then check again, 2-3 days later for example, it can easily be 50-60+, and my computer is a few weeks old, so it's not a bad one.
If you got a old computer, your fan might be full of dust, this resulting in a bad cooling. You should check your computers temperature when you start HB, then when you come home what it is at.
GL!