PTEX rendering ?

So i'm experimenting - I quickly painted Gen3 in mudbox using PTEX, exported the ptx

In daz I loaded genesis3 and used to obj exported from mudbox to load new uv in Daz and then loaded the ptx into the diffuse channels

Now, when (in 3delight) I render a medium size image I get squares appearing, but if i render a larger image all is ok
attached 2 images(the larger resized for the forum comparison

totally new to ptex so trying to find out a bit, anyone know much about using it with Daz Studio, or if the above method is right or not?