uneven power delivery, or could be dirt in the tooth of a gear, i have had a smallest piece of shit in the tooth of a main spring barrel stop a movement and that is where the movement has the most power (but moves the slowest) it has the least power to over come an obstuction at the escape wheel but moves the fastest.
If the main spring is dirty then the spring will bind and stick and you see big flutuations in the amplitude, but hat can be cased by dirt and old oil in the rest of the movement as well.