Q: it has a misfire when at slow speed

If the symptom seems to clear up at higher speeds, this may be related to the idle air control valve which monitors air/fuel ratios at idle an lower speeds. This is a unit that is located on or near your intake manifold that monitors the air intake as it is mixed with fuel prior to being injected into the engine. This valve is controlled by the vehicle’s computer which will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed. However, when the engine RPM drops below the normal range of 800 RPM (for most cars), it is more than likely that you are dealing with an IAC valve problem. In cases where this valve is clogged or is nearing its replacement age, your vehicle will stall instead of idling after the release of the throttle. I would recommend having an expert from YourMechanic come to your home to replace your idle air control valve.