it is possibe not to have any new cavity or root problem for 8 years if you are maintaining very good oral hygeine. some people they do not have any cavity or any other dental problem for years. it depends upon the genetic factor, salivary factor and so many other factors. if your dentist has already taken a X-ray and do not find any abnormality then it is o.k regarding intermittent pain it can be because of your teeth wear that is why your dentist has asked for the night guard. you may be grinding your teeth at night. if you want to know more about the bruxism and the night guard you can visit the following link ...... http://www.identalhub.com/articlecategory-teeth-grinding-bruxism-5.aspx