my 770 is from expansys uk too. no problem so far, been using maemo mapper - havent test it with a gps retriever though. if it starts acting strange after installing something (maemo mapper for example), u can try to uninstall it
(also delete all maps) and then install again. change the path of the maps (in maemo mapper) to your mmc instead of using the internal memory as the mapsize can get very large.
you can install the system-monitor-plugin to monitor the cpu usuage, etc. which i think is helpful.