Multiple.img files from CloudMade

Has anyone had problems with loading multiple .img files to his GPS device?
I am travelling to Germany soon, and it seemed a perfect opportunity to download and install France, Belgium, Netherlands and Germany .img files from CloudMade on to my Garmin Legend HCx. From what I can determine, individual countries cause no problem, but if I create a gmapsupp.img from more than one country .img file, there is a lot of ‘noise’ in the form of straight black and red lines criss-crossing or horizontal when I view the area on the GPSr. The total file size is 262Mb, so no issues there, because I have a 2Gb micro SD card.
I should welcome any advice on getting all of these countries to display properly.

If I’m right then the cloudmade maps make unnamed roads more visible then others, so if you just want to cruise around then you might be better off with another source.