That looks like your original curves might be pretty messed up - check to see if they are not full of micro segments and other noise. Turn on control points and/or explode the curve to see. You may end up needing to completely re-do them, FitCrv might possibly be a semi-automatic solution. If you can post one of the offending curves here, someone can have a look.
Hi Alex - from the model you sent it’s hard to say what went wrong at this stage, without the input objects, but my guess is one or more Boolean operations was involved and the surface edges got out of whack- to fix it, ExtractSrf each of the surfaces that has the crazy edges and isocurves and delete, then Loft between the upper and lower edges of the resulting gap.