[lowfighter]

Asheshouse on map_T paint your last pixels at the borders of the map with same color, say lowland0 color. Although I'm not sure it's this because you work in excelent mode and there might be other problems...
To be more precise see what happens at the northern border. If the last pixels are forest for some distance then the forest will be extended off map to north indefinitely. If last pixels for some distance are lowland2 for example then the lowland2 texture will be extended off map to north. Etc. Generaly all the standard maps take into acount that and there's only one texture which you see off map.

I have a recurring problem of squares of texture popping in and out of view at certain viewing angles.
This seems to only affect areas close to the map edge where the land/sea boundary is running off map.

Tried playing around with map_T land/sea boundary but cant get rid of it completely.

This screenshot shows the problem. You can see the squares in the distance. As you move the viewpoint around the squares disappear and reappear.