Without knowing the particulars about how he constructed the map I can't be sure, however we had to overcome some similar problems with the desert map. There is a hard coded "roughness" associated with each texture type. If you get the types mixed around you can run into this. For instance, "terr004.bmp" is intended to be a rock texture. It has an extremely rough bumpyness when driven over. Which is logical if it is a rock texture. However, if the map design decided he didn't need a rock texture and wanted to reuse the texture number for say open field, then you still have the predefined "roughness" associated. Eventually I believe HT intends to make the roughness assignable to the texture the way you can assign the clutter type to the texture. Until then you have to be careful and anticipate which textures will get the most gv traffic and assign the best texture number for that. If I remember correctly the order best to worst for gv traffic was:
Best: terr0001.bmp (grass)
terr0016.bmp (snow)
terr0003.bmp (farm)
terr0005.bmp (forest)
terr0002.bmp (grassy rock)
Worst: terr0004.bmp (rock)
I'm not sure if this was what he was running into. Hope it helps.
Regards,
Wab