Sounds like your having a similar problem to the one I'm having with my 570di. the depth finder seems to get stuck between 16'-18' depths while the chart is updated uninterrupted. It flashes as if it has trouble locating bottom. It only flashes between these depths and will do it from 10 - 20 minutes at a time.
The warranty is up on mine so its going in for repair.
http://www.humminbird.com/Faq.aspx?TaxonomyId=95A1764