Just adding two screens for direct comparison, to make sure it's not mistaken as purely a user-side graphics issue:
Custom wall on the left (seam), Maxis on middle/right (seamless):
http://www.modyourpanties.com/hosting/62200_141021151536seam_custom-vs-maxis.jpg
Maxis on left/middle/right (completely seamless):
http://www.modyourpanties.com/hosting/62201_141021151607seam_maxis-seamless.jpg
I have a relatively lame graphics card (but am within specs) .. and your walls look seamless, so HOW are you doing this? o.O
Also here are some direct links to packages in case that helps:
http://www.modyourpanties.com/hosting/62202_141021152846pbox_simplesiding_left.7z
the one in my screenshots above (single tile left edge)
http://chii.modthesims.info/getfile.php?file=1474495
not pictured above, screenshot
here
