It looks like the only corrections the car can make are full right turn or full left turn. The car only detects two conditions, the white line is under the sensor and it turns away from the line. You can try a little wider line of consistent width.
If the car can go straight, you can add another condition, and maybe follow the line closer. How about making your line wider. If both sensors detect the line, it goes straight. When one sensor looses the line, it turns back to the line?
Dave