Hello I have a Lexus sc400 and sometimes it's having problems that come and go.

What is your question?

My car is acting strange it's done it before. It's like the car has a mind of its own. I used to put cheep gas and it started to shake and exhaust smoke would pour in my cabin. So I changed to premium after the mistake I made. So it ran great for a month but just recently it started again it would stall and shake and smoke would come in the whole 9 yards. The check engine light is not on so what's wrong? I love this car but I can't figure out the problem. Your suggestions and comments are greatly appreciated. Thanks