Steel Girder Diaphragms vs. Cross Frames

I am new to LEAP Steel (and 3D modeling in general) and am trying to analyze a single span curved steel plate girder bridge built on a 214' radius.  The span length along the centerline of the alignment is 80'.  The cross section will consist of 4 plate girders with 27" webs.  Due to the shallow nature of the girders I will need to use diaphragms (channel or w-shape) in lieu of cross frames.  When i run LEAP Steel in grillage mode, it runs fine with no problem.  When i switch to finite element mode the structure becomes unstable.  I then switched the diaphragms to K-frames (did not change anything else) and it becomes stable with a finite element analysis.  Does anyone know why this may be?   When I move the bridge to a tangent section of roadway it also runs without any instabilities.  

Thanks for the help,

Anthony