Usually it starts good, but when put in gear, it is rough and when you start going, black smoke comes from the back. Sometimes the check engine light comes on, but usually will either go out or won't light when you start it the next day. Sometimes it will go into a fit and stall when I stop, then has a hard time starting, runs very slow for a couple minutes, then speeds up and I can go then. If try to go when it is running slow, it instantly will stall. When It is running slow, if I touch the gas, it will stall too. When driving with the light on, it will go out and run perfectly for a while. Usually when I take it to the shop, it is running good and they can find nothing wrong. Where should I start checking?