Hi Barry, yeah a loft between 2 long bendy curves will give a result like that. The generated surface will be formed by connecting equal proportion distances traveled along each. If you turn on the lofted surface control points you'll see the sort of twisting effect that is making your unwanted curvature:



To get a surface that has a line at those particular stations a couple of things you might try would be to loft between only 1/2 turn pieces so they would be synchronized at those areas or possibly do a sweep instead of a loft. Or if you are generating it procedurally maybe generate a lot of the line cross sections how you want them and then loft between those lines rather than lofting between the long paths.

- Michael