--Is this a known problem with TYPs, line definitions with borders and 
--(certain?) Garmin GPS units or am I doing something silly?

It's the first time that i see this effect.

There are two ways of giving a definition for the drawing of a polyline in typ file.
1. providing a bitmap definition
2. providing the definition in linewidth/borderwidth format
For further informations you can have a look in the manuals of the mapcompiler cgpsmapper and MapTK.

I don't think that the effect is only caused by the way of definition.
But maybe it's caused by erroneous definitiopn in the typ file and/or the map or it's related to the gps unit.
First i would have a look into the typ-file.
Afaik not all Garmin Gps have the ability to use typ-files(especially older series doesn't),
so may be this also could be a reason


cheers 
Gert