Technical issues with Heroes III.

Hi. I have a question. I have a desktop and a laptop, both running Windows 8.1. The game runs fine on the laptop, but on the desktop, it constantly lags, freezes, and doesn't keep up with mouse movements. The desktop is much more powerful than the laptop, but it's impossible to play the game on it. Maybe I need to adjust some settings? I've tried running it with different graphics settings, but it's still the same.
Update: I noticed that it freezes when I move the mouse. All animations stop while the mouse is moving, even the hero's movement. And for it to work properly, I have to stop the mouse or hold it over an object.