Tangency in surface loft

[dixib]

Hi

I'm trying to create a surface loft (the image below shows the loft without the existing geometry).

I've almost made it work now, but am still having some trouble. When trying to merge my loft with the existing geometry, there are a lot of small gaps and one too large for the knit future to heal. The profiles are converted from the existing geometry and the guide lines are sketches projected onto the surfaces of the existing geometry at the ends of the lofted profiles (with three intermediate guide curves). In this way the existing geometry and the loft should match perfectly, but apparently doesn't...
When zooming in close on the end of the loft, there's a strange tangency problem (blue is my loft, grey is the existing geometry). For at small part of the loft, the tangency shifts. The rest of the loft along the guide curve has the right tangency direction. Can anyone explaine why the tangency changes direction along the guide curve?

All help is appreciated. Unfortunately I'm not able to supply a model.