This morning, I started my car, and it ran really rough for about a minute. There was some white smoke coming out the back. The problem went away, and everything seemed normal. This afternoon, I tried to start it, but it turned over, then died. When I tried again, it stayed running, but ran rough again, and I saw more white smoke. Again, it ran fine after that. My coolant has been disappearing, with no leak. Do I have a bad head gasket, or could something else cause this? Would that also explain some of the misfires I've had around 3000 rpm?