By design between LOD changes there are discontinuities (holes), and skirts hide these holes.
However if they are not there, then the holes will be visible.
Oh wait! I think I know what's going on. GIMP seamless is not producing what you need. It is indeed seamless, but not what you need (it is off by one).
The problem is the following: let's assume a 8x1 texture, GIMP will produce the following:
This is indeed seamless. If you put two 8x1 textures together in a row you will get the following:
However this is not what you need. What you need is the following:
Notice that the 0 in the middle is repeated. (and that now the 2nd texture ends in 2 instead of 0). That is what you need. The borders need to have the exact same value.
At least I suspect this could be what's happening.
Cheers
Matias