AutoCAD Civil 3D :: Break A Closed Feature Line

I often use feature lines for parking lots and ponds.  They're great when they work, but they drive me NUTS when they don't - which is a lot of the time.  For parking lots and ponds, my feature lines usually close back on themselves.  That's when I have trouble breaking the feature line.  When I use BREAKFEATURE, I choose two points and this should, in theory, leave a gap in the feature line between those two points.  That's not what happens.  I end up with a huge gap between the first point I picked and a vertex point 5 or 6 vertices away.  It doesn't matter where I pick the breaking points.

I've also tried TRIMFEATURES.  I draw lines across the feature line where I would have chosen the breaking points.  But the portion that I want to remove is the portion that always remains.  It doesn't matter where I select the feature line to be trimmed (inside or outside of the cutting edges).

I think there may something wrong with the feature line itself.  Sometimes, if I erase the original and draw a new one, it will trim just fine.  But that's such a headache, and it doesn't always work.  If I knew what was causing the feature lines to go squirrely,  I would aviod it.  Unfortunately, it seems like breathing the wrong way can screw up a feature line.