This could happen when you first start out targeting your keywords. I think some people call it the Google dance. If your site / business is fairly new, then don't worry too much about this, it should settle down soon.
Meanwhile, it's best to diversify your back linking strategies and avoid spammy techniques (i.e. link exchanges and irrelevant comments). Write unique content related to your niche and post to blogs and article directories. Ping them and submit to social bookmarking sites. Also build links naturally by getting involved in relevant discussions, forums and comments.