Interesting, noticed the same problems with textures of 3D objects, didnt report it, cuz i could track it down to one command.
Maybe its related with the possibility to draw your texture multiple times with just one call, if you use texturecoordinates <0 etc...
to see what i mean use a image with size 64x64 and draw it with this polyvector:
STARTPOLY circle(you can do the same with textures for 3D objects)
POLYVECTOR 64,64,-64,0, 0xffffff
POLYVECTOR 64,512,-64,128, 0xffffff
POLYVECTOR 512,512,64,128, 0xffffff
POLYVECTOR 512,64,64,0, 0xffffff
as you can see, you can draw 1 sprite multiple time with only one call (dont know if this is intended/feature or bug). Maybe this +smothshading causes this lines, that you found.
BTW you can get rid of those lines if you set SMOTHSHADING FALSE. At least for polyvector, but it wont solve the problems for Textures of 3D objects.
Maybe someone should write a bugreport about this?