Humanoid:Move() doesn't move at a constant rate

I’m trying to make a game which, in short, involves Humanoid:Move to move NPCs. However, after about 15-30 seconds of the game, the NPCs spontaneously increase almost 8 times in speed. There seems to be no correlation between other events in the game and the moment at which the characters speed up. Is there another way I can achieve the same effect as Humanoid:Move/MoveTo, or am I doing something wrong?

thanks :>