I was able to solve my problem. I was using mapsource patch and jetmouse generator that were found in a torrent. They seemed to be identical to the ones that were posted to this site, but apparently they were not. I did the whole installation again with the mapsource patch and jetmouse generator that were posted to this site and what do you know: when I copied the new IMG map file to the sd-card in my GPS the map works superbly!
Thank you for the files found in this site and thank you for your advice Catymag!
P.s. If this thanking post is useless it can be deleted.