Heavy Lines on OSM in Basecamp

[Herbivore63]

I'm new to using Basecamp and a GPS with tracks. I've just managed to import an Open Street Map into Basecamp and it seems to be working fine except that there are heavy diagonal and vertical lines showing up making it almost unreadable. I've been through all the Basecamp settings, googled everything I can think of and still can't figure out the problem. I'm hoping someone can help. I'm also hoping that the screenshot link below works as I've never posted an image on here before! Thanks for any help...

http://herbivore.smugmug.com/Other/U...%20lines-M.jpg