erratic

This term describes a lack of stability or predictability, often carrying a negative connotation of unreliability or instability. It is frequently used to describe mechanical failures, volatile financial markets, or unstable human behavior, suggesting a jarring departure from an expected norm.

In a behavioral context, it implies a sense of chaos or lack of control, whereas in a physical or technical context, it suggests a malfunction or an irregular pulse. It differs from "random" in that erratic behavior often fluctuates around a baseline rather than being completely devoid of any pattern.