Yes, that fits exactly to our finding! The J1 heatbreak is the problem! 
With that PTFE lined hotend you circumvent any issues with heat transfer and early melting of the filament simply by using the PTFE liner. This is the reason why such hotends (such as the E3D Lite6) are still sold and frequently used by the way - they are easy to use and very forgiving. The downside however is a reduced melting speed and a reduced max temperature.