Why does print fall apart at beginning of top layer?
I doubt this is a printer or filament issue. Rather, I suspect it might be related to how your slicer handles bridging scenarios (which basically is what laying down layers over infill is).

You could try to:


Increase the infill percentage and/or use an infill pattern that provides better support for the top layer (so that it isn't allowed to sag as much). You will probably still se some sag, but that is usually covered up by the next layer. Adding print cooling might also help here.
Adjust the bridging settings in your slicer software. If under extrusion is the main issue, perhaps you could try to increase extrusion for bridges?


I am no expert with Simplify3D, and I suspect that the default settings actually might be pretty good. I would therefore try option 1. first. Perhaps someone else here can give a better answer related to your slicer. :-)