diagonal lines on my Garmin/OSM

My garmin/osm map has diagonal lines going across it, the only way I can get rid of them is to reduce detail in Basecamp

Do you mean something like this?

That is how the basic garmin map looks if the map doesn’t come with a typ file.
You can download a generic map with a typ file or use the generic new style that comes with a typ file.
See http://wiki.openstreetmap.org/wiki/Garmin.OpenStreetMap.nl#Screenshots