Slope is a property of 3D linear geometry, and I guess that you have calculated it based on elevations of consecutive two points.
If so, I think you can also create line segments connecting between every pair of consecutive two points and add the attribute "shape_class" (Uphill, Flat, Downhill) to the line segments, rather than to the points. You can then get desired result by connecting the line segments with the LineCombiner (called LineJoiner in older versions) grouping by the "shape_class".