Hi, I have a 97 318 convertible with just 44000

Hi, I have a 97 318 convertible with just 44000 miles. Used very infrequently. Last 6 months or so when started the RPM would go up and down and the motor would chug and bounce in neutral. Once in Drive, it smoothed out under acceleration. Check Engine Light on. a few days ago it sputtered at startup while I was near a garage. They scoped it and said a record on the cam sensor. For the past two days it started up with just a little of the problem and the Check Engine light is now off. I did put in some Techtron cleaner a few days back. Does a cam sensor problem just disappear or is this a lull before the storm? What does this sensor sell for? Installation costs? Location of sensor? Thanks