I tried it with slope = 5 as well. The artifacts appear at the edge of the screen when scrolling and on the south slope as well. With a flat top, the building spaces icons on top of the mountain also create graphics artifacts.
Maybe this is related to the dithering of terrains. While creating terrain patterns sometimes i forget to index the image (with indexed images they loose their transparency). The result is quite surprising Some things (don't know what) gets painted on the transparent area and while scrolling these things get repeated in the transparent area.
The dithering is made with a mask (file: /world/pics/edge.png) with lunate form. Maybe with too much slope on the mountains the angles of adjacent terrains gets to obtuse/sharp and the mask does not cover all adjacent terrain patterns. Just a thought.
I played already with the mask and tried to make it a bit more smooth, but i haven't found a better form then it has now. Changing the dithering will cause great code changes i believe.