Trouble with surface split giving strange outcome

All,

I have some trouble with the surface Split function. Somehow it does not do what i thought it should do.

In my model i have Four vertical lines, which i divide in set pieces. Here i connect the points on the same 'number' again. 

This fluent lines i then divide and close with polylines (this so in the total model the shape can be more or less fluent). These lines I then loft to have a closed volume. If I now want to cut the bottom halve of this, it results in very strange leftovers. See attached files. There are no two lists created but many more.

Anyone who can see what i do wrong? 

Thanks

Tijl